If you're into that kind of stuff, go deep sea fishing and make sure you find a boat that lets you keep what you catch (if you guys cook that is). It's a ton of fun and some of the best fish you'll ever eat.
Go hiking, get off the map a little...but overall, relax and have fun. Hawaii is a great state, I usually only go to Maui, but Kauai is beautiful as well.
Have fun!