Honestly, I would stay away from Oahu. It is way too crowded and traffic is a nightmare. There's nice sights here and Pearl Harbor obviously, but any of Kauai, Maui, or Hawaii island would be your best bet to really experience Hawaii. Depending on how long you would plan to stay, you might be able to hit two of those islands. Inter-island travel is fairly affordable and quick. Any of those three islands should also have decent luaus if you look for them. Almost all of them will be commercialized, but not nearly as bad as on Oahu. Kauai is the Garden Isle and the most green of all of them and Maui is also beautiful and both have great hikes. Big Island is nice, but I would personally stick to the other two for a shorter trip. Diving, shark cage diving, skydiving, whale/dolphin tours, fishing, helicopter rides, surfing, paddleboarding, bike tours, and plenty of other outdoor things are pretty easy to come by on any of the islands. I like paddleboarding with the sea turtles for a good relaxing workout here.