The top things to do in Hawaii are:
1. Go to Pearl Harbor to see history at the US Arizona Memorial. You can drive or take the bus there; no need to book a tour. (Oahu Island)
2. If you are on the island of Oahu (where Honolulu and Waikiki beach is) go to the other side of the island to see the great deserted beaches (get a rental car for a day or two).
3. If you go to the Polynesian Cultural Center (Oahu) understand that its run by the Morman church and there are NO alcoholic beverages served there, not even beer. I say skip it and do a Luau instead.
4. If you go to Maui, you gotta do the downhill bike ride. I don't recommend the sunrise one because you have to get up at 3:00 AM to get started up the mountain and its cold up there that early. If you are a romantic do it, but otherwise no. There are different bike companies and some don't take you all the way to the top, and some don't guide you back down. You can come down by yourself, there's only one road with just a little traffic, but if you are squimish about riding in the street, a company that accompanys you down will help watch the traffic for you. I have been twice and prefer Maui Downhill to the others. These companies have web sites They will pick you up at your hotel, take you to their shop to pick out a wet-suit (for the cold and mist/rain shower maybe) and gloves and helmet; then take you up the mountain; then follow your group down so they can block traffic for your ride; then afterwards take you back to your hotel. Expect this activity to take 75-80% or more of the day.
4. Also in Maui, The Road to Hana is a great drive (you need a rental car). Its a 30 mile winding road that takes 4 hours to drive. Its like driving through the jungle; has a bunch of one lane bridges.
5. On the Big Island of Hawaii: go to the Volcano National Park. Also go see Akaka Falls.
For information about tours and things to do, click on the link below for Robert Hawaii. They do tours on all of the islands.