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To quote DH, Maui has more things to "do", Kauai has more things to "see". It is not as built up as Maui, iirc, no hotel is higher than 3 or 4 stories, so it maintains it's pristine beauty.
Our faves - Oahu - since you have to fly in through there anyway, may as well stop and see Diamondhead and Pearl Harbor. Diamondhead is a fairly short hike and the views at the top are awesome! Pearl Harbor was awesome too. We just stayed one night there. Waikiki is very built up, but once you drive away from it, you can check out the northshore and the pineapple plantations.
Maui - Haleakala, watch the sunrise at the top, take the bike ride down (optional) the Road to Hana (start early in the day, it's very windy and takes a long time) - amazingly beautiful. It's a common thing to do, but due to the numerous stops, it never felt crowded at all. Many stops we were the only ones there.
We also went scuba diving when we were there and took a plane ride over the big island to see the active volcano.
Kauai - Waimea Canyon (looks like the Grand Canyon but not as big), hike the first 2 -3 miles of the Napali Coast (it's amazing - I remember DH and I went a bit off the beaten path and wound up on a secluded beach with caves), a great way to see all of Kauai's beauty is by helicopter. I totally recommend doing it, you get breathtaking views of Napali, Mount Waileale with it's waterfalls, (I think I may have spelled it wrong), and Waimea Canyon. Beaches are so pretty, we just drove around the whole island and checked out all of the coasts.
Honestly, on all the islands, there is so much to see that we actually didn't want to waste our day laying out on the beach.
I think we only did that one day, iirc or maybe another 1/2 day. We were there for 12 days total, and since we live in FL, we can lay out on beautiful beaches anytime, lol.
I don't know if you're going with kids or not...it does stink to be in the back seat of a convertible as your head feels like it's being blown off, but we were there on our honeymoon so hence no kids yet, and a convertible was a must!
I personally loved all 3 islands. They were all different in their own way. But all very beautiful. Kauai is the most pristine, Maui more built up, Oahu way more built up since it's very populated.