If you want to snorkel, Hanauma is the best. But go early, because it does get really crowded. I think it has the best snorkeling in the world. You could then climb up Diamond Head if you wanted to or continue around to the back side of the island. There's a ton of stuff to do if you have a car. I've snorkeled off the reef across from the shopping center (Mauna Loa?????) and a couple of places on the back side, too.