Everyone is different and that's why your question is so hard to answer. My first suggestion would be to go to your local library (or bookstore) and check to see if they carry the "Revealed" series - there's one for each island. That will help you decide what you might want to see or do on each of the island. Usually, people explore the Waimea Canyon or the Na Pali coast (via a helicopter) on Kaua'i, water sports or a trip to Hana or up to Haleakala is big on Mau'i. The Big Island, there's Kona for water sports, Hilo, there's Volcano National Park. Honolulu (on Oah'u) has aquariums, museums, Pearl Harbor, Diamond Head, the Dole Pineapple Plantation, the Polynesian Cultural Center and on and on.
Do you prefer water sports, hiking, nature, shopping, fishing a combination of two or more?
My biggest suggestion is don't try to do everything. Pick out one or two things and stick with that. It's better to just do a couple of things and really enjoy them then trying to cram everything in and not remembering any of it. Hawai'i is all about relaxing.