There is so much to do on Kauai, few people even get the chance to see 10% of the magnificent places this island has to offer. While a lot of use like to spend our vacation time at peace on a beach, for those of us who crave a bit more activity Kauai is the place to be...
For starters, there are tons of Parks. From Kokee State Park and Waimea canyon on the West side to Haena on the North Shore there is a variety of free hiking and walking trails located around the island. Beyond these state parks there are a variety of companies that offer tours into some of the island's backcountry lifestyle. One of the biggest is Kauai Backcountry offering a wide variety of tours on innertubes, quads, ziplines, and other adventures that fit your fancy.
Outside of the tours, there is a variety of other options that are suitable for non-beach days. If it is raining on Kauai which happens pretty frequently you might consider a Train Ride on one of the original sugar cane plantations on the Island. The tour takes guests through the history of Kauai and its storied agricultural history. This tour is also a bit easier on the knees than some of adventure tours out there.
Whatever you decide, just remember Kauai's backcountry has boatloads to offer than other tropical beach destinations do not hold a candle to!