You'll find answers to your question on a similar thread...
http://www.tugbbs.com/forums/showthread.php?p=617215
Here are my thoughts on things to do on Kauai, organized and otherwise:
Ten things that you should definitely do, but, not necessarily in any order other than the order that they pop into my head.
1. Helicopter tour
2. Tour the Na Pali by kayak or one of the tour boats.
3. Go to the end of the road and enjoy Ke'e Beach for a day and stop along the way every time you get the itch.
4. Go to the end of the road in the Waimea Canyon and stop every time you get the itch.
5. Hike at least part of the Kalalau Trail.
6. Visit Queen's Bath... and jump in... but be wary of high surf!
7. Visit SEVERAL beaches that you have to walk a trail to get to... not only will you enjoy the sand and surf, but you will enjoy the feeling that you are on a deserted isle... this is one of my favorites!
8. Either kayak, ski, or take the tour boat up the Waimea River.
9. Get into the interior of the island... hard way... hike. easy way... take the Movie Tour even if you don't like movies (there is the coastal tour and the interior tour... go interior).
10. ATV it... there are several tours... if you are up to it, take them all.
11. Stroll Hanalei.
12. Stroll Koloa
13. Tube down an irrigation ditch... no? seriously! it is a blast!
14. Visit the Botanical Gardens... all three, but at least the best one... the one down by Spouting Horn... you'll see a bit of "Jurasic Park"
15. Hike some more... take your pick of trails
16. Eat a Puka Dog... even if you hate hot dogs... just do it! You will thank me.
17. Have lunch at the Polynesian Cafe
18. Visit the Light House and hang around to watch the sea, surf and birds.
19. Zipline through Princeville Ranch Adventures (notice I named names this time... because their zip is the best.)
20. Start over and do your favorites all over again.
Two more that I can't believe I forgot... two great natural swimming pools... Kipu Falls, where the brave can jump in from a rope swing... and the falls seen on Fantasy Island,, Waimea Falls... where many adventurous types climb down a rugged trail to jump in the pool at the base of the falls and some even make the climb to walk behind the falls. (Fair warning, I have only broken one bone in my body and it was at Kipu Falls... but I go back every visit.)
There are many more things to do, including some obvious activities like surf, sun bathe, snorkel, etc. I know I listed more than ten, but I listed things that I consider to be musts. If I think of some more, I'll add them.