If you want an adventure on Kauai, do the secret tunnel hike. There is a mile-long tunnel running through the mountain. They used to send water through it from the Hanalei river to irrigate the sugar plantations. There are actually as many as 5 tunnels like that in the middle of the island. We only got to the first one coming from the west.
Hawaii is pretty damn sweet. Was there a few years back with my wife and we stayed on the big island. We checked out the volcano natl park which was pretty sweet. We also did a kick ass guided waterfall hike and got to walk through a 500ft waterfall. We did do a bit of snorkeling like everyone always suggested... my wife loved it but I came face to face with a big ass eel which scared the crap out of me