The easiest way to enjoy the water on Lanai is to wade in at Hulopoe Beach and swim or snorkel. If you prefer an organized excursion, a fishing trip is a good bet (you keep some of the fish). Snorkel trips are a great way to see the island, above and below the surface, and scuba divers can marvel at one of the top cave-dive spots in the Pacific.
Manele Golf Course will certainly test your skill on the green. Experienced hikers can choose from miles of dirt roads and trails, but note that you're on your own—there's no water or support. Remember that Lanai is privately owned, and all land-based activities are at the owner's discretion.