You need a rental car on both - there is no good way to get around the islands without one (other than Oahu)

Yes you need a rental car on any island except Oahu and even there it's nice to have one at least part of the time. Try this site: www.discounthawaiicarrentals.com - they rent from all the majors. Do not wait to book your rental car as the rental car companies reduced the number of cars in their fleets 2-3 years ago.

Been a long time since I was on the BI, in Maui I would stay near Kaanapali or maybe south in Kihei (cheap condos there) or Wailea as we went in November one time and stayed near Napili and got some rain, so do not stay further north that time of year.

Great outdoor opportunities on the Big Island - and check other posts in here under Hawaii. Hiking around the Volcano is great, check out the Ranger talks/walks, and Akaka falls North of Hilo, etc.

Also lots to do on Maui besides go to the beach (catch the sunrise up at Mt. Haleakula) - but you really should try snorkeling. Rent the equipment for a week (check out the Snorkel Bob's of this world - http://www.snorkelbob.com/cgi-local/SoftCart.exe/online-store/scstore/hotpicks.htm?E+scstore and you can take the stuff to the next island and turn it in there) and practice in the hotel pool before going in the ocean.

Be sure to keep any loose hairs out of the mask as it can let a drop of water leak down and it becomes maddening when it's on the lens.

Also use the fins in the pool to get used to them and most important - practice how to also breath with your face in the water. It will open up a whole new world.

Generally - go early in the morning when it's usually the calmest on the ocean and it has the best visibility.

Weather could have some storms blowing through - but then it usually clears up and is warm.

For a reasonably priced place, look for condos or apartments to rent on the Big Island. They book up early for the holidays, so get after it now! Try VRBO.com, airbnb.com, TripAdvisor.com. Or if you really want to stay in a hotel check out the Marriot Courtyard in Kona - the King Kamehameha,. Also look for Ourigger resorts properties. The have the Keauhou Beach Resort Hotel in Kona, which is a nice, clean comfortable place and not terribly expensive. these two are right in Kona. Do NOT stay at Uncle Billy's! Old, rundown, crumby place!

You absolutely have to have a car!

Thanks a lot for all the information and suggestions! You guys are really helpful!
With 9 days in total, how shoud I split it between Big Island and Maui? Anu suggestions?

I would consider 2 days up at the Volcano/Hilo side - and say three on Kona side - and then kick back 4 days on Maui.

Two questions:
1. I'll arrive at Kona and plan to fly out from Hilo. Do all car rental companies allow me to drop the car at Hilo? With some extra charge?
2. Do I have to buy the flight ticket from Hilo to Maui in advance? How many flights are there per day from Hilo to Maui and what's price roughly on Dec. 21?
Thanks in advance!

1. I checked Alamo, Avis, Hertz and the Costco websites--all allow pick up KOA - drop off ITO--no additional charge. (BTW Costco was the cheapest (thru Alamo)-if you're a member) You can also try http://www.discounthawaiicarrental.com

2. Hawaiian has more than 10 flights ITO-OGG on Dec. 21. Prices range from $98 for the one non-stop leaving Hilo at 8:24 am. Prices for the other flights (all of which require going through Honolulu) range from $116-$160. A few of the mid-day departures are already sold out. Since the day you want to fly is right before Christmas, I would buy tickets in advance.

martym,
Thank you sooooooooo much for the detailed information!
It helps a looooooooooot !!!

We like Sea Village condos on the Big Island. They are VRBO for the most part.