Hawaii is an amazing place. If you have the money and a license, you owe it to yourself to go SCUBA diving. The water is so crystal clear that you can see for thousands of feet while underwater. They have HUGE turtles (bigger than you), and all kinds of aquatic life. I got to go diving there when I visited when I was 13 and its possibly one of the most remarkable experiences of my life.
I need to get certified first. I tried scuba once in a swimming pool and ended up with a hideous ear infection, so I need to check that that was a one off before risking it on holiday. But just snorkling was good, which somewhat surprised me..