It's hard to go wrong between Maui, Kaua'i, and Hawai'i. Each has plenty to see, and you could spend 8-9 days on one island alone (though I get bored after about 5)

We've been to the islands three times in the last six years that the count is Oahu - 1, Kaua'i - 1; Hawai'i - 2; Maui - 3. Next trip will certainly include Kaua'i and I think we will do 2 of the 3 each two week trip (excluding Oahu) going forward.

I'm glad we went to Oahu - we had to go to Pearl and see the Arizona Memorial, Big Mo, etc. - but probably won't be back for a while. Not that it's bad - it's like having a date with the 4th prettiest girl at the Miss Universe pageant, but the top three are waiting for you

If you're looking for active plans for a few islands, check under my history for a post from about 5-6 years ago. I posted an itinerary that got a universal "are you crazy, you'll be exhausted, just relax" response, but we did all but one of the items and it was perfect. Of course we always manage to put 3-00 miles/wk on a rental car on any of the islands, Aruba, Grand Cayman, St. Kitts, etc.