Ever since this 70-square-mile spit of land was purchased in 1864 by Elizabeth Sinclair, a wealthy Scottish farmer, Hawaii's Forbidden Isle has been most famous for being off-limits. But the wild landscape of arid, red-tinged volcanic terrain is easier to reach than you think: you can still take day trips from Kauai. Join Niihau Helicopters, which will land you on secluded beaches with nothing but shells, translucent water, and a few endangered monk seals ($400 per person; niihau.us). Or scuba through 100-foot-wide lava tubes with monk seals, spinner dolphins, Galapagos sharks, and humpback whales (three-tank dives from $315).
Access: Trips start and end in Kauai. The 17-mile crossing takes 2.5 hours by boat. Stay at Koa Kea (from $346).