Waimea Canyon is awesome. Its a neat drive and when you get to the top the scenic overlook doesn't get much better. I would recommend getting there early as the fog has been known to roll in around noon (or so I have been told.) Similar to WOY we stayed on the other side of the island. They offer boat tours on the Na Pali Cost that I have heard are amazing. My wife and I booked one only to have it cancelled.
Be very careful with your belongings, our car was broken into at one of the popular tourist locations. RedsFan74 sounds exactly like our experience, my wife's bag was stolen in a smash and grab. Thankfully her wallet and license was not in it.
The seafood is amazing and different than what you can get on the east coast. Drink plenty of Mai Tai's, drink those fufu drinks while in Hawaii.
Buy this book. This was our guide in Kauai. http://www.amazon.com/The-Ultimate-K...keywords=kauai