We love snorkeling and this trip we tried Snuba for the first time and we loved it. Poipu Beach, Lawaii Beach, and Ke'e Beach were the best places we saw for snorkeling.
Taking a drive to the far north side of the island (we stayed in the south) was definitely worth the trip. You pass through all different kinds of scenery and when you get to the far north it becomes like a tropical rain forest. Be sure to check out the town of Hanalei -- do some shopping, eat lunch, get a shave ice, and then head on down to Ke'e Beach.
The other great thing to do in Kauai is EAT!!! Have lunch or dinner at Keoki's and be sure to have a sunset dinner at The Beach House.
Quick Tips:
The best tip would be to buy
The Ultimate Kauai Guide Book. This is a great tourbook on Kauai and really helped us pinpoint what to spend our time on.
Be sure to bring good hiking/walking shoes (like Tevas). Many beautiful places can be seen by a five minute walk through dirt or somewhat rocky terrain. We had our flip flops on and still went to these places, but I wish I had my Tevas.
If you go to Queen's Bath (which I definitely recommend), be SURE to go at low tide. We went at high tide and were not able to swim. It looked like so much fun that I definitely want to hit that spot again when we return to Kauai.
Also, hike (five minutes) to Kipu Falls and swim in the refreshing lagoon and try out the giant rope swing. Very fun!
Best Way To Get Around:
By far, convertible rental car. It's worth the extra expense!