We were there September 08 and stayed in the Poipo area. Our experiences are focused on the south part of the island.
Jack Harter does a no door tour.
http://www.helicopters-kauai.com/
You must do the no door tour from whatever company you pick. Must. Just pay the money and go, don't ask questions. They DO NOT have a canned recording. You have 2 - way communication with the pilot the whole time.
Waimea Canyon - go hike some of it
Na Pali Coast - The weather can sometimes be unpredictable and if the waves are up I don't think most of the boats go. If you happen to be staying on the north side then you can hike to the beaches fairly easily.
Snorkeling with turtles - I can't remember the name of the company we went with on our snorkeling tour but the leader was excellent. She is a marine biologist so she knows something more than "they're green". Really good tour, nothing fancy. It was just a smaller boat (not a cat) so it might be a little wavey at times. I'll try and search for the card at home.
Diving - Scuba Steve is the man.
http://www.scubasteveonkauai.com/ . He's a bit of a goofball at first but his attitude is infectious. I found him online and sent him an email to book and he phoned me back to confirm everything. I was pretty damn impressed with that alone. He also has a unique Hawaiian Green Sea Turtle PADI specialty course which he runs. Highly recommend if you're divers, you'll never look at turtles the same way again.
If you want specifics it seems there are a few of us who could answer. And lastly, my pics:
http://www.stadevene.com/kauai.html