I've done ATV tours on Oahu and the Big Island and both islands were great for ATV tours. The Oahu tour takes you to the location of where Jurrasic Park was filmed, bamboo forests, and some nice vistas high in the mountains. I agree with EveryAnn that Hanauma Bay is great! as is the North Shore which is a nice drive. Renting a car in Oahu is the only way to go. Its cheap and will allow you flexibility in gettting around. You might consider a catamaran cruise as well.
If you looking to do alot of activities and save some money check out the
Power Pass it includes free admission to 25 top attractions in Honolulu like Dole Plantation Maze, Battleship Missouri Museum, Kualoa Ranch, USS Arizona/Pearl Harbor Tour, Sea Life Park, Iolani Palace, The Bishop Museum, Hawaiian Waters Adventure Park and lots of discounted products.
I'll see if I can dig up a few photos of the ATV tours and post them.