I spend a year on Oahu in the late 80's (interned at the Shipyard) and got to visit 6 of 7 islands. My fav - the Big Island - most laid back, widely varying terrain (even had snow on the top when I was there), black sand beaches, and I got to see the lava flowing into the ocean.
One thing the tourist books rarely discuss is the hiking. Every island has great hiking from the coast up to the base of the interior mountain where spectacular waterfalls are created from the rain constantly falling around the peak. I got maps from the local government centers. They will warn you to be careful as the weather in the higher areas is cold and can change rapidly leading to exposure. I don't know if it is still true but the locals warned me about wandering into someone's 'crop' as well since the rain and rich soil are good for growing legal and illegal plants.
Another big draw for me is the food - get as much variety as you can since almost any pacific rim food can be found in Hawaii. I didn't find any local (hawaiian) food on either Maui or Kauai but if you can find it, Kailua pig, pork lau lau, lomi lomi salmon and, of course, poi are great. Other local favs are malasadas, chorizo, manapua, spam (the stuff from ham) and sno cones (taste great at the beach). The hotel restaurants had macadamia nut pancakes with coconut syrup. The sushi was really fresh too.
Finally, the outdoor sports, especially those involving wind (windsurfing the trade winds) and water (snorkeling and boogie boarding) are highly recommended. I never did the helicopter thing but the glider ride I did on Oahu was great.
Maui was mostly for windsurfers (great winds) and tourists (shopping). The road to Hana was a great drive and people who've done it speak highly of the sunrise bike ride down the mountain (Haleakala).
Kauai's had the quietest beaches and some of the fanciest resorts. I don't know if they are still there but the resorts were outrageous - lagoons, zoos etc. I never made it to the mini Grand Canyon there but it is supposed to be quite scenic.
My info maybe a little dated but . . .

Pau Hana!! - RG