Snorkeling on the Napali coast, who watching tour, helicopter tour of the island, hiking in Waimea canyon, golf, surf lessons in Hanalei Bay, and the list could go on. Lots and lots of hikes and outdoor activities on Kauai.
Whatever you do, don't miss a stop at Chocolat Hanalei (in Hanalei) and get some Passion Fruit Domes.