Kauai is very informal: You'll get by with shorts, T-shirts, and sneakers at most attractions and restaurants; a casual sundress or a polo shirt and khakis is fine even in the most expensive places. Don't forget a long-sleeved coverup (to throw on at the beach when you've had enough sun for the day), rubber water shoes or flip-flops, and hiking shoes and several pairs of good socks if you plan to do any hiking. You might also want to bring binoculars for whale-watching.
Be sure to bring sun protection: sunglasses, strong sunscreen, a light hat (like a baseball cap or a sun visor), and a canteen or water bottle if you'll be hiking -- you'll easily dehydrate on the trail in the tropic heat. Experts recommend carrying 2 liters of water per person per day on any hike. Campers should bring water purification tablets or devices.
Don't bother overstuffing your suitcase with 2 whole weeks' worth of shorts and T-shirts: Kauai has laundry facilities everywhere. If your accommodations don't have a washer/dryer or laundry service (most do), there will most likely be a laundry nearby. The only exception to this is Kokee Park, so do a load of laundry before you arrive.
One last thing: It really can get cold on Kauai, especially if you are staying in Kokee. It's always a good idea to bring long pants and a windbreaker, sweater, or light jacket. And be sure to bring along rain gear if you'll be in Kauai from November to March.
