The best thing to do here is hike. There is so much beauty on this island that you have to hike to see. If you're happy with driving around and seeing the sights then you won't truly see what Kauai is all about. Snorkeling is incredible, but be careful where you snorkel, the currents are so strong that if you're not a good swimmer then you'll be sucked out to sea and be shark food. The luau's are also a must see in Hawaii, because it's part of the culture and the Smith Luau was the only one I saw but it was amazing. Be Careful in the Napiu(SP) Mountains if you're hiking there, some pretty crazy people live in those mountains and haven't showered in months. Don't forget lots of water wherever you go hiking because you need it to survive. Stay away from the boat tours if you don't have a strong stomach. We did the 7 hour tour that included alcohol and food, about 20 of the 27 people on the boat were sick the whole trip. Everyday is a highlight in Kauai, and it's a great adventure as well.
Quick Tips:
Sign up for a discount card at a local grocery story, I saved a day on food and alcohol. Best Way To Get Around:
You need a rental car on the island because it's too hard to get around. A lot of people were hitch hiking around, because they didn't have cars.