ABC lol
If you like snorkeling, go to hanauma bay.
http://www.hanauma-bay-hawaii.com/
You can take a bus (cheap). Go early in the morning and ask the driver when the last bus leaves. (important). Its very busy there, the snorkeling is better earlier than later.
As well, on the way back, ask the bus driver to let you know where the costco is, if you have a card. Cheap booze. Cheaper than duty free. Or ABC lol a close second.