Any hints on must see destinations on Maui or Kauai? We are a young couple (25-31) and enjoy hiking/walking. We will have our own car as well. We will be based in Wailea on Maui and in Poipu on Kauai.

Pick up two great guide books for your islands. Maui Revealed and Ultimate Kauai Guidebook. Both are well written, entertaining and informative. They list activities, beaches, hikes, etc.

Preview them at wizardpub.com

On Maui, be sure to do the Road to Hana. It sounds kind of silly to just do this drive, but we really enjoyed it. Pick up one of the tapes they sell everywhere -- and put it your rental car player, it will say "At mile marker #17 (or whatever was coming up) pull over to the side of the road and walk 20 yards down the path..." and there would be these amazing waterfalls and all kinds of beautiful scenery. The road winds along the coastline and was great for pictures... You don't have to go the entire way to make it worth your while.
We also enjoyed the Maui Ocean Center, and had dinner at the Waterfront close by there.
Have a fabulous time!

I second those guidebook recommendations.
On Maui, hiking in Haleakala's crater. On Kauai, kayaking (or, a boat ride from one of the smaller operators) off the Na Pali coast, hiking in the "grand canyon," snorkling at Tunnels Beach, walking on the beach at Polihale (though the rental car co. will not want you to take the car down the access road, so be careful driving).

You should do a search on the forum since this is a very well-covered topic here. (I'm going to Maui myself in 11 days and my BF and I are also in our late 20s/early 30s. This board has been an incredible resource!)

On Maui, you'll get plenty of hiking opportunities off the Road to Hana. Also, you can do a hike on the Haleakala crater. (There's also the downhill bikeride we're considering doing and horseback riding in the crater as well.)

The snorkel cruise to the Molokini crater is another popular thing that people do. (We're going scuba diving instead.)

Have fun planning your trip!

Doc04, the guide books everyone are talking about are very unique and very good. They are fun to read and very thorough without feeling like you are reading an encylopedia. What I realized after we arrived was the maps and directions in these books use mile markers and you won't believe how helpful it is to know "okay, we just passed mile marker 17, the book says 'turn just after MM 20'". So you know to be looking for your exit in 3 miles. ALL of the other tourist maps we picked up at the stores, airport etc are so full of advertisement that it's hard to find where you are heading/how long it will take.

Each time we took off on a road trip, I'd read the little stories out of the book - they were pretty amusing and the have nice dead on reviews for most of the restaurants too. Online you can get reviews of all the hotels if you still need those.

Here were my favorite things to do on the islands you mentioned:

Kauai:

Helicopter Ride (this eliminated are need/desire to drive our car up the canyon since the helicopter went right down into it)

The hike at the end of Ke'e Beach. If you don't do any other hike in Hawaii - please do this one down to the beach and back. It's a good 2 hours each way and some time to relax and watch the waves at the beach. This trip was beautiful and worth the day.

We took a Napali Boat trip which was fun, 'cause we wanted to get out on the water and enjoy the coast fro there.

Eat shave ice at JoJo's.

Maui,

Eat at Mama's Fishhouse

Drive up to the Crater (we went midmorning and stopped for lunch on our way down. If you are going to Maui first then you may want to do the very early morning bike trip down (you leave from your hotel by van at about 2:30 am) so if you are still on Continental USA time -the first morning would be a good time to do this.

We did not do the road to Hana because Kauai was breathtaking and it's the best. We talked to some people who were dissappointed in this drive because it is so long and tiring. But othere like it. Maybe someone here on the boards has done both and can compare the 2.

Hope this helps.