Get a copy of Kauai Revealed. A casual day can be floating down the old sugar cane irrigation system which includes going through several tunnels that were hand dug. Zip line in Koloa. The Kauai Coffee Planation. A day trip to Waimea Canyon. Drive all the way to the top, hike around, have a picnic lunch, and stop at the View Points on the way down. If you like casual dining the Chevron Station in Princeville serves some of the best Chilipepper Chicken on the Island. The Fish Taco Truck by the Hanalei Pier is very good. The Snack Shack at Sueoka's Market has great Loco Moco. There is the Rum Distillery just outside of Lihue though the Rum is cheaper at Costco. Costco is also the place to buy gas. Our favorite drink is 1/3 Koloa Dark Rum and 2/3 POG juice. You need to have a drink or lunch at Brennecke's at Poipu. For swimming or snorkeling there is Anini Beach, Ke'e Beach, Tunnels Beach, Poipu Beach, Koloa Landing, etc. Hideaways Beach is also good Snorkling. But to get there you climb down a cliff or Kayak from Hanalei. Bring lunch, water, books, etc. There is the Makaauwahi Cave. The Mahaulepu Heritage Trail.
If you are in good shape there is the Kalalau Trail that starts by Ke'e Beach. This is more like climbing not a simple hike. Trek Sticks help. After two miles you get to Hanakapiai Beach, where they recommend you do not swim. Then it is a 2 mile climb/hike to Hanakapiai Falls. It is gorgeous. You ford the river 3 or 4 times. Wear hiking shoes but bring water shoes to swim in the pool. Also carry lots of water and a lunch. This 8 mile round trip took us 8 hours, which included an about 30 minute break at the Beach and about an hour break at the waterfall. You will need sun screen and bug repellent. Or you can use the juice from the Ginger Flower for both. We were both 60 when we did this hike. Though i have to admit Patti spend the next day with ice bags on her knees.
If you haven't guessed we love Kauai and go there every year or two.