Ke'e beach, Kauai. The road from the west side of the island ends there, and after that, it's all hiking trail. Gorgeous, rugged, and a little wild - many people have been killed by the surf, and if you get seriously injured, the only way out is by chopper (which we actually saw in action, unfortunately).
Sunset from Poipu, Kauai. Situated at the southern tip of the island, Poipu offers great snorkeling, swimming, and relaxing. Although some areas are more commercialized, it is great to find a little corner that's a little more out of the way and has a great view!
Kalalau trail, north shore of Kauai. The 11 mile trail runs from Ke'e Beach to Kalalau beach, with a few intermediate stops like Hanakapi'ai Beach (seen here). It can be a difficult hike with constant changes in elevation, exposure, and weather that varies by the minute. Pro tip: at the beach, the combination of strong currents, changing tides, and isolation makes for regular drownings - stay out of the water!
On the island's rough northwestern edge, inaccessible by car (only hike, boat, or helicopter). Verdant and gorgeous.
The "Grand Canyon of the Pacific", red rocks and all. Very, very wet, in slight contradistinction to the one in Arizona.
Queen's Bath, Kauai. If the surf is calm, the water becomes fantastically clear, revealing the underwater flora and fauna. In this natural pool carved from a lava shelf, there are areas for jumping, snorkeling, and enjoying the magnificent scenery. Be careful though! The rocks are slippery, and if the surf is not cooperating, you may be swept out to sea.