I think Pearl Harbor / Arizona Memorial is a must-see for anybody who has never been to Oahu before. It's really moving and I think even small kids get the significance of the area. Other things that are good/fun to do - the hike to the top of Diamond Head (go early in the AM before it gets hot!), spending the day at Polynesian Cultural Center and learning about various cultures of the Pacific, going out to Hanauma Bay to snorkel with fish (or to Shark's Cove if it's not wintertime), the views at the Banzai Pipeline and Waimea Bay of the surfers if the surf is up during the winter (though traffic will be a mess). Just some thoughts.