Well, besides the standard beach activities (sunning, swimming, surfing, snorkeling, scuba lessons, etc.)... On Maui, you can visit the top of Haleakala, go on a sunrise bike trip down the mountain, take a drive out to Hana. Definitely check out the Old Lahaina Luau -- great show, good food, and all the drinks you can handle :) On Oahu, you can visit the Arizona Memorial (Pearl Harbor), the Polynesian Cultural Center and the Bishop Museum. On Hawaii (the Big Island) you can see the active volcano (Kilauea) and visit Volcanoes National Park, go to the top of Mauna Kea (just under 14,000 ft. elevation). I think some of the islands (if not all) have some sort of ropes course, or zip line activities. You can go on a horseback riding trip, go camping, visit old sugar cane or pineapple plantations, go parasailing, hiking, ride a bike, or just walk around town and shop. If you know what island you'll be visiting, just go a google search with "things to do on _______" (insert name of island) and you'll get lots of suggestions. For some really popular things (Like the Old Lahaina Luau on Maui, or the Polynesian Cultural Center on Oahu) you may want to buy your tickets ahead of time so you don't get there only to find out they're booked full. Most of the specific things I mentioned above have websites, you only have to search for them. You can usually buy tickets or make reservations online. Most of all - relax, and have a good time.