I was just there for my honeymoon in Dec. Hmmm where to start.
Definitly hit up a plane tour with "Wings over Kauai"; its just a dude and his wife, he's a former biologist and he has so much cool info on the island. Its cheaper than a heli tour and wayyy more personal. It just seats you 2 plus the pilot, and he's talking to you the whole time. All the heli tours are just canned recordings. Hell, he even let the wife do all the requests with the control tower!
http://www.wingsoverkauai.com/
Aside from that there are a lot of things to do; where are you guys all staying?