Hi Whickey, and congratulations on your upcoming marriage.
There are so many wonderful things to do in Hawaii. On Maui, we've taken the van ride up to the top of Haleakala, a dormant volcano at 10,000', and bicycled back down again. We've kayaked in Kauai. We've visited Volcano National Park and watched people throw chickens into the dormant volcano as a sacrifice. We've rented a car on a couple of islands and visited beaches, went snorkeling, walked on lava that recently covered a road, walked on black sand beaches. We saw waterfalls, did some shopping, saw a rainforest.
There are zip line rides all over the place, lots of hiking, waterfalls -- the real question is, what do YOU like to do? There are a lot of things offered from sightseeing on a bus to much more active activities, golf, fishing, etc. Whatever you like to do, you can do it in Hawaii, in gorgeous surroundings.