Darren, my wife and I are headed to Kaua'i next month as well... 9/12 - 9/17. First visit to the Islands for us. When will you be there?
We are going biplane flying with Kevin from Tropical Biplanes (Google it), he is based at Lihue.
For "what to do", check out The Ultimate Kauai Guidebook by Andrew Doughty--written by a pilot who lives on the Island. I grabbed the latest Kindle edition off Amazon, or you can grab the iPhone app if you use Apple devices.
We are going snorkeling, an ocean catamaran tour to Na Pali from the north shore with Na Pali Sea Breeze, the obligatory Lu'au (Smith Family Garden Lu'au at Wailua Marina State Park), the biplane flight, some easy hikes and some relaxing.
TripAdvisor.com also has good information and reviews from travelers, for free.