Here in KAUA'I, there is virtually nothing to do at Nawiliwili, where the boat docks. It isn't a very nice part of the island and is primarily industrial. You are stuck and captive. Perhaps the best way to get around would be to rent a car for the day (no, not moped). The car rentals will pick you up. Then you can have your choice of Waimea Canyon, Hanalei and the north shore, or a plethora of other choices. If you keep the car overnight, it costs $10 to park near the boat, although you could leave it in the nearby county park (questionably worth it, and not advisable late at night).

PS. Cabs in Hawaii will cost you a $$$ for short distances. Seriously, we were at Kahalui recently and went to the mall (nearly walking distance), perhaps a mile away. The metered fare was about $15 in each direction. Reminded me of taking a cab in Germany or Switzerland.