Rent a car and head out to the North Shore, just a gorgeous drive and a great way to spend the day. There's also a great nature park (whose name escapes me atm) near the North Shore that you can spend many hours in.
Or.. if not wanting to drive, even just walking around the area of Waikiki Beach is a fun time. Lot's of cool shops and hole in the wall restaurants. Many people say it's too crowded and touristy - and it is - but imo it's still a very enjoyable place to visit.
Have a great time, wish we were going too!
