Depends on what you're interested in.
For cultural stuff, Polynesian Culture Center can't be beat. Spend an afternoon/evening in Waikiki just to see/do the tourist thing. There is always the Arizona Memorial, as well as the Missouri. Olomana golf course is fairly easy to get on, and reasonable on green fees after 3pm. It's the same course that Michelle Wie learned on, and it's very nice. Drive the H3 over to Kaneohe Bay, it's a VERY scenic drive that takes you right through the mountains. Lots of nice beaches on the North Shore, most notable being Waimea Bay. Across the street is Waimea Falls Park, albeit it's pricey to get in. You can stop by the Dole Planatation tourist stop on your way out to the North Shore. There is a really fun water park, Hawaiian Waters Adventure Park. It sounds like a touristy thing to do, but actually, it's close to the opposite. When I talk to my family, they're all "tired" of the beach, and they like to go there. Go figure.
There's tons of stuff to do, you'll have lots of fun.