I've only been to Oahu so....

There's a famous dive shop off Northshore, I can't recall the name but if you make your way there you can't miss it. They can set you up with what you need.

China town can be fun if you like shopping.. and if you're decent at bargaining. ^_^

The open air mall downtown is definately worth visiting. There's an italian restaurant on the bottom floor that serves Tang for dinner too. :P

Whatever hotel you're staying at will most likely have all that information available. I would suggest the beach, some hiking and more beach.