Kauai is a beautiful island and, although it is fairly small, it is packed with things to see and do. There is a lot of great snorkeling, a miniature Grand Canyon (Waimea Canyon), a miniature Old Faithful (Spouting Horn, a blow hole where waves frequently shoot water 50 feet into the air), and it's the only island with rivers you can navigate down in kayaks.
Before You Go to Kauai we'd recommend getting the Kauai Revealed book. You can get it here. That is hands down the best guide book we’ve seen for any destination. You’ll see other people with the book all over the island. The book will highlight things to see like waterfalls and it will give detailed instructions on how to get to the highlights.
If you enjoy snorkeling, we highly recommend buying your own snorkeling gear, you can do so here. It will be cheaper than renting gear and in a place like Kauai, there are dozens of great snorkeling spots you can do free if you have your own gear. You just go to the beach, put on your gear, and hop in the water.
Kauai if pretty small, you can drive "around" the whole island in a day, but you can’t drive around the whole island. One patch of island, along the Na Pali Coast, does not have roads. The road ends on one end at Ke’e Beach and ends on the other end at Waimea Canyon. Both of these bookends are HIGHLY recommended. Waimea Canyon is awesome, and Ke’e Beach is one of the most beautiful beaches in the world. Because you can’t get from one to the other without driving around the rest of the island, before you go you should map out what you want to do each day so you don’t waste time driving back and forth.
One unique thing about Kauai is the rivers. We recommend renting a Kayak for a few hours and navigating a Kauai river. Before you go you should reserve a kayak so you can be sure one is available for you.