There is so much of the outdoors to experience on Kauai that taking a guided tour may not be such a bad idea. You will find, however, that with Kauai, it's not a matter of what you decide to explore there, but how.
Hiking
The two big hiking attractions on Kauai are Waimea Canyon State Park on the west side, which is dubbed the "Grand Canyon of the Pacific," and the Na Pali Coast State Park on the northwest side. Both hikes provide memorable views of some of the most scenic natural wonders Hawaii has to offer. Though there aren't many guided tours of either park, the fun is in discovering each for yourself.
Pick an Adventure
Kauai Backcountry Adventures has several fun and unique tours of the island's forests. If you enjoy a good adrenaline rush, the zipline tour sends you soaring like a bird over valleys and rivers. If you prefer to keep your feet on ground (or water), their tubing tour is just as fun, coasting through tunnels built during the plantation days of Hawaii. Regardless of what you choose, the experience is guaranteed to be unforgettable.
Another popular touring option on Kauai is via helicopter, since some of the island's natural wonders can't be reached via foot. At about $200 a person, you and your party can see Kauai in ways otherwise impossible, complete with info from a knowledgeable guide/pilot.
Mahalo Suggests
When planning your trip, also see Kauai Vacation, Kauai Hotels and Kauai Restaurants.