I often receive emails from visitors to Kauai looking for personal recommendations on things to do while on island. Here are some suggestions.
There is a lot of natural beauty on Kauai to explore! I recommend driving all around the island (one day the north shore, one day the south shore, and one day Waimea Canyon on the west side). By renting a car and driving around the island you will see a lot of great sights relatively inexpensively.
Here are some of my favorite things to do starting on the north shore: Ke'e beach or Tunnels beach, having lunch at Bubba's in Hanalei along with some shopping, Hanalei Bay (this is a lovely 2.5 mile crescent shaped beach with majestic mountains with waterfalls in the background - you might have to pinch yourself to believe you're not in a dream), Kilauea lighthouse and nature preserve (not open on Sundays), and Banana Joe's fruit stand.
East side: Blossoming Lotus restaurant in Kapa'a (great vegetarian food and world known), Smith's Tropical Paradise Luau in Wailua, Smith's boat tour up the Wailua river to the Fern Grotto (you will be serenaded by live Hawaiian music and hula dancers), Coconut Marketplace for shopping, Eggbert's for breakfast, and Dinner at Duke's Canoe Club restaurant at Kalapaki (great salad bar, fish, chicken and reasonably priced for an 'ocean view' restaurant).
South and west side: Poipu Beach, Lunch or dinner at Brennecke's (wonderful food, good prices, and ocean view), Spouting Horn, the "Keiki" Hula Show at the Hyatt (free, usually on Tuesday & Saturday eve, call first), Brick Oven Pizza in Kalaheo (best pizza I've ever had - closed on Mondays), Kukuiolono Park and Golf Course (breathtaking views of surrounding area), Kauai Coffee Visitor Center (free coffee samples of all of their varieties, self-guided tour of the coffee process, largest coffee plantation in the US - Harvesting to start in mid Sept.), Grinds restaurant in Ele'ele for breakfast, lunch or dinner (affordable), Salt Pond Beach, Waimea Canyon (all the way to the Kalalau Lookout), Waimea Plantation Cottages for lunch or dinner (brew pub and walk on their lovely grounds to the ocean), Polihale beach (bumpy road to get there and hot, but very picturesque), and of course a visit to my Waimea art studio, Painting Paradise :-)
Also, I recommend taking a boat tour to see the island from the ocean. I personally do NOT receommend a helicopter tour (there have been too many fatal crashes). The helicopter tour is nice because you get to see the whole island, but it is risky. Also, be very, very careful in the ocean. I am sick of hearing in the newspaper about people who get swept out to sea and drown. This happens all too often. Swim in protected areas, be aware of the current, and talk to a lifeguard before going in, if possible. Kauai is paradise on earth, but accidents still happen.
You can get more info about the island by reading the local newspaper: http://www.kauaiworld.com/ , or http://www.kauaipeople.com/ .
If you need to book a tour I recommend http://www.kauaivacationtours.com/
(I know the ladies who work there. Tell them Marionette sent you.)
If you want to shop for local products made on Kauai, check out the "Kauai Made" website at http://www.kauaimade.net/ . This program was created by the County of Kauai to officially represent the products made on Kauai by Kauai people, using Kauai materials.
If, after all of this, you are a little tired and sore and in need of some TLC, then I recommend Tropics Day Spa http://www.tropicsdayspa.com/ located in Port Allen.
If anyone has more suggestions of activities, sights to see, or restaurants, please list them as a "comment". I'd love to see what your favorite things to do on Kauai are!