The popular snorkeling attraction in Honolulu is Hanauma Bay. It is a Nature Preserve (no fishing or spearing) and consequently the fish are not afraid of humans and are rather plentiful. You can rent snorkel gear at the Bay but note they are closed on Tuesdays and there is an entry fee.
The Aquarium in Waikiki is also worth a visit as the tanks are well stocked with all types of marine life.
If the waves are big you might want to take a drive to the North Shore to view the surf and visit Haleiwa town. Traffic may be a problem but its worth the visit.
I will 2nd Morbo's suggestion on visiting Byodo-In Temple. It is located in the Valley of the Temple Memorial Park and as stated it is very beautiful and stunning.
Enjoy your visit and don't leave valuables in your car at the different scenic spots.