Transit between islands takes about 4-5 hours total ( from leaving your hotel on one island to arriving at your hotel on the next island). If you leave by 7:00, you'll arrive and be "ready to go" by noon at the latest. Based on that, I think what you've planned is very doable, but whether moving around will bother you/stress you out, etc, is obviously not something I can advise you on. It also depends what you're planning to do on each island. For example, if you plan to visit Pearl Harbor, Waikiki, the north shore, Diamond Head and Kailua on Oahu, 4 days is plenty of time. If you plan to add in the Doris Duke estate, Bishop Museum, Dole Plantation tour, kayaking, shopping, etc, then you'll need more time.

"Transit between islands takes about 4-5 hours total ( from leaving your hotel on one island to arriving at your hotel on the next island"

But, you also need to pack and unpack, check in and out of hotels, return and pick-up rental cars. It is a hassle.

For three weeks, I would do one week each: Hawaii, Kauai, Maui or Oahu (depending on the vibe you want). Lucky you! You really can't go wrong in Hawaii. I have never heard anyone say they wished they hadn't gone to any of the islands.

I don't consider it a hassle, but the OP might.

I consider it a hassle when I could be spending time enjoying myself vacationing instead of spending 3/4 day in transit. IMHO.

Thanks all. Peaceout; Are you saying you'd choose big island, Kauai, and either Maui or Oahu?
I was considering skipping Maui, but still can't decide.

September is a great time to visit but with 3 weeks I'd stick to 3 islands, especially Big Island which ideally needs 8-10 days since you need to stay in either Hilo or Volcano Village and then move to Kona or Kohala coast for the beaches and touring that side of the island.

I love Kauai and the North Shore would be gorgeous that time of year.

Part of the reason people love to go to Hawaii is the vibe. You will miss the Aloha if you don't slow down and get on Island Time. This is not a 'checklist' sort of vacation, as you may have in some cities around the world that have lots of history. September is also the hottest month in Hawaii. LOL

You can do a bit of a 'checklist' visit if you take the NCL cruise that departs Honolulu every Saturday afternoon. Then you'd stay on one or perhaps two of the islands for your last two weeks. The cruise is not deluxe, but it's OK and the only rational way to glimpse the highlights of three islands in a week.

*I* would skip Big Island in September. It's BIG. It's HOT. You need to stay in two locations to enjoy both west side and Volcano National Park. Come back for this.

So...now you have time to *enjoy* the three smaller islands!

Forgot to ask if you're traveling alone or with spouse/S.O./family.

Also, if you're coming from the East Coast, you'll find best air in and out of HNL -- so that's one of your islands. I look for one long leg nonstop on a 767 or 330. I avoid 757's and west coast connections.

If you like to move around - no problemo. Actually - I say you can get from one hotel to the other in about 3 hours - but that may be because we know how to do it. We can leave the hotel and usually be at the airport in under an hour, and check in and take the flight (the Inter-island airport at Hono is smaller than the huge International airport - and the outer island airports are also fairly small) - another hour/second one all told and then an hour on the next island to pick up your rental car and get to your place.

As for packing and unpacking - how much stuff does one take to Hawaii? You need a swimsuit - two perhaps for the ladies, and about three different cotton outfits, maybe a sport jacket for a guy and a pair of slacks - and of course, another outfit or two/three for the gal.

We can get by with carry-ons for a week to 10 days (includingor swim-masks) and if it is a longer trip - perhaps check a bag with that stuff, and maybe a second bag if we take our dive gear.

If you had to cut one island out - consider perhaps Oahu- or maybe Maui - wherever they have the most direct flights from where you live - because you might decide to go back on a lower cost package deal to one island - and Oahu by far has the most such "deals" - and Maui has some - but again, it all depends on where you live and how direct the flights are.

For a suggested 4 island trip - you can see/preview Oahu in 2 to 3 days - or at least get a good feel/drive around the island - even going part of the way - after say you snorkel at Hanauma Bay - and then be off to the next island the moring after.

I would of course recommend a week on our favorite Isle - the Garden Island of Kauai.

As for between Maui and Big Island - decide if you want more night life and beach time - Maui has a lot of nice beaches both up at Lahaina/Kaanapali/Kapalua/Napili - or down at Wailea/Kiehei - or if you like more diversity/driving around to see sights, including the incredible Volcano and going hiking, etc - choose Big Island/Hawaii.

Spend one week at the one where you think you will enjoy more of what you want to do - and then spend four or five days at the other one.

As you probably know - there are many other posts/advice in here - under Hawaii.

Wow tough question!!! I've been to Hawaii 5 times so far and only made it to Oahu and Maui to date. Just depends what you want to see and do. Big Island is... well... BIG and very diverse (not totally tropical or what many people expect to see in Hawaii).

love BI. BUT we generally just plop ourselves down on beach and perhaps snorkel. It is big but very unique as compared to the other islands. Kauai and BI are the best. 13 trips so far to the islands. Don't care for Maui so 2 trips there was enough for us.....

Hope you can work out all the details. Hawaii is awesome and so much fun. Something about palm trees, crashing water on the lava rocks that is healing for the soul.....

With 21 days I would split up the time to see all 4 in 21 days

Big Island - this island takes a week (or 7 days) as its the largest of the two. its best to split your time by staying near Volcano National park and then Kona or north of Kona (some very nice resorts about 20 - 30 miles away from the airport.

Maui - it depends if you want to see the Road to Hana or not as it can be a full day exhausting experience to do so. If you just want to see the Western section; get to top of Haleakala one day; and so forth, you can do Maui in 5 to 6 days.

Kauai - smallest of the islands, and possible to see most of it in 3 days. Drive one direction from your resort one day; the second drive the other way from your resort. Then one day just to relax and enjoy.

Oahu - it really depends on you as Oahu the city is just that - a city by the ocean. If you are a shopper and enjoy late night outs (not just the usual island luau or dinner for example) its where you do it. Personally I have been in Oahu and now just spend a day or so there as I live in a large city already. As such I am looking for the slower island style life and big waves of North Oahu. For North Oahu and the Polynesian Cultural center, one day shopping in Oahu, I would put your minimum stay in Oahu at 5 days.

Flying between islands is an hour flight, but as mentioned there is the coming and going to the airports which means it is a 4 or 5 other affair as the other poster said. So just calculate 1 full day of your 21 days will be in transit....

As such

Big Island 7 days
Maui 5 days
Kauai 3 days
Oahu 5 days
In transit (although you will still have time to explore an island) 1 day

That adds up too your 21 days.

If you want more beach time, you will want to just do 3 islands.

Aloha and enjoy.

We just came back from Hawaii and let me tell you, I was trying to squeeze in as much as possible and thought about skipping Maui. However, I'm so glad we didn't because it ended up being our favorite island!!! 3 weeks is plenty of time because we just did all 4 islands in 12 days. I have been to Hawaii before though so it was a little easier to plan certain things.

We did
4 nights in Maui
3 nights in Kauai
3 nights in the Big Island
1 night in Oahu with a another full extra day being that our flight left at 9pm.

We had enough time in Maui, could have used another 1/2 day but we saw so much; Hana, Haleakala, West Maui Mountains, Lahania, Kahuli, and a little of south Maui. I would have liked another 1/2 day to spend in South Maui and relax.
Kauai was fine but we chose our last day to visit Waimea Canyon and it was so foggy. If we had done it in the beginning, we could have left and came back. The Marine life preserve was closed so we found ourselves wandering along the North Shore, which was beautiful but far from where we stayed (Poipu) so it was hard to enjoy knowing we had such a far ride home. I would have liked another day there to just relax on the beautiful beaches.
The Big Island was way too big to do in 3 days, it was not possible to see more than 1/2 the island. We stayed in the South (Punalu'u) which was so far from Kona and the north side. We were near Volcanoes National Park which allowed us to visit several times. I loved the beaches in Kona though and wish I had another day to spend there. We also missed the whole North Shore which I have heard is beautiful. 5 days minimum in the Big Island to see a little of each part. 6 or 7 you've be able to see all the main areas but moving around a lot. 8 would be more relaxing but I personally don't think I would have wanted to say there that long. It's very hard to get around the Island which I'll explain later.
Oahu, we were exhausted by the time we got to Oahu so we stayed at our resort but we did visit Pearl Harbor and Waikiki for sunset. I had been there before so I wasn't super concerned about seeing all of it. In my previous trip, I visited the North Shore and Hanuma Bay so we skipped it this time. However, I would have liked 1 more day in Oahu.

The traveling between islands was fine, we just made sure to see a few spots on the way from the airport. In some cases, this worked out well if the airport was far from our resort.

I was not able to take off more than the 12 days but we had an amazing time and saw so many beautiful sights! Coming from the east coast, I wanted to see as much as possible.

If I had the 3 weeks I would:

6 nights in Maui and stay 1 night if you can in Molokai (ferry or small plane)
(You will not regret staying this long in Maui!)
6 nights in the Big Island
5 nights in Kauai
4 nights in Oahu

Even though the Big Island could use more nights, it is very hard to get around the Island as there is ONE main highway. If you don't mind driving a lot, spend 7 nights in the Big Island and only 3 in Oahu.
I would not give up a day in Kauai for the Big Island - Kauai is more lush, green and relaxing but the Big Island is more adventurous & rugged. Depends what you are into.

If you can fly directly into Maui and then leave from Honolulu, that works well.
FYI:
Some inter-island flights require a stop in HNL from Kauai to the Big Island so check on that.
Your clothes will get muddy in a lot of these locations so be prepared to spend a night or 2 doing laundry.

Everybody seems to have favorite isles - and ours are Kauai (especially the North Shore) - with Big Island a close 2nd, but we love them all.

To me - I would spend the first couple/3 days on Oahu - kicking back a bit from the flight and getting to know Hawaii a bit - including of course, snorkeling at Haunama Bay ( http://www1.honolulu.gov/parks/facility/hanaumabay/underwatertour.htm go early to beat the crowds - and overall - I think it's still the best snorkeling opportunity in the islands) - and then decide where you want to end up and plan accordingly.

For instance, if you thought at the end of the trip you wanted to wind up mostly on the beach - after Oahu - maybe you visit beautiful Kauai first - as it's only a 20 minute flight - and about the time the flight attendants serves your beverage - the Captain is putting on the fasten sea belt signs - in prep for landing.

And do take advantage of a very clear day to get up to see the Grand Canyon of the Pacific - as Mark Twain called Waimea. And taking a Catamaran ride/sail up along the incredibly beautiful Napili Coast is also very special, and hopefully you will see dolphins and turtles as we did, and while we went scuba diving from the boat - they also offer snorkeling and with great visibility in the water - that's also a lot of fun. http://www.napali.com/

Also - at the end of the road on the north shore - is our favorite beach - K'e (Haena Beach Park - http://www.fodors.com/world/north-america/usa/hawaii/kauai/review-429904.html with great snorkeling - unless the surf is really kicking up - so don't fight it - but that's rare in summer) and from there - if it hasn't rained too much that morning - it's a beautiful hike up along the hill - overlooking the Pacific. Some however think it's a bit challenging so ask around/read up on it before you go, but we thought it was a blast.

Also read Song-doc's great posts about the Bali Hai experience of North Shore, Kauai.

Then perhaps take the longest inter-island "commute" you would have and head to Hilo on the Big Island, changing planes on Oahu at the smaller Inter-Island terminal - which is easy to navigate. If the flight comes down the east side - you get a beautiful view of the Waipo valley, etc.

From there - you can easily visit Akaka Falls, http://www.hawaiistateparks.org/parks/hawaii/index.cfm?park_id=2 and then easily get up to Volcano for a couple days and wind up on the Kona side - and then fly to Maui for a long beach week somewhere, and if you have time on Maui - take the ferry from Lahaina to Molokai or Lanai. http://www.go-lanai.com/ It's a beautiful ride - and you can also arrange for snorkeling, whatever.

Or - mix it up however you choose.

And yes, we don't take too much in the way of clothes, and may do a load or two when over there - but hey - just mix up a nice Chi Chi http://www.letstiki.com/home/2011/11/13/how-to-make-a-chi-chi-cocktail.html or Mai Tai to enjoy while enjoying the view and waiting to transfer the stuff to the dryer.

We just returned from 11 days on the Big Island, and I am not sure that was enough. Unlike some others, all (my, wife, 13 yr-old son and 8 yr-old daughter) of us prefered the Hilo side way more than the Kohala side. If I never go back the the stretch from the south end of Ali'i to Puako, that will be just fine.

There is so much to do on the Big Island. You can surf, kayak, hike, visit water falls, eat some incredible food, visit some neat cultural museums/sites, snorkel, star gazing at the obsevatory, and hang out at the beach. I would not feel compelled at all to spend less time on an island and visit more islands. We also did 8 days on Oahu last year (staying on the North Shore), and we felt like that was not enough.

With 21 days I'd spend most of the time on Big Island, relax on Maui for a week and then add a few nights on Oahu OR Kauai depending on the vibe you want if you must.
More relaxation? Kauai. More nightlife Oahu.
I like Oahu but not nearly as much as BI and Maui and would be happy with 2-3 nights there.
Spend more time if you like to surf.
For me 2 islands would be enough for 21 days, 3 would be OK but I wouldn't want to move around any more than that.

10 days on BI minimum. It's big and plenty to do. One of my favourite places.
I think you do need to split your time here and stay on both sides of the island.
Ideally a week on the Kohala coast and say 3 or 4 nights in Hilo or near the volcano.
A week on Maui would be wonderful but you could get by on 5 nights.

I've not been to Kauai yet..

Kauai has the most luscious greenery/rain forest type - especially on the North Shore - hence the name the "Garden Isle". While you can kick back and enjoy the scenery - there is still plenty else to do there.

My Q: If one hasn't been to Kauai - how can you comment about a suggested 3 or 4 island trip not including it?

Just a quick reply, we started our trip in Maui and did everything BUT relax. We had 3 1/2 days (daylight) to see as much as we could so if you want to relax, 5 days are needed minimum.

I felt Kauai (first) and Oahu (second) were the most relaxing of all the islands, just my opinion but Maui was still fantastic and adventurous.

<<Tomsd
My Q: If one hasn't been to Kauai - how can you comment about a suggested 3 or 4 island trip not including it?>>

How can I comment? Just like anyone else I guess.
Start a new thread with your question Tom if you want anymore answers!