Volcanoes National Park
Camp in the park at the southeastern tip of the island, and hike out as far as the rangers will let you go. Kilauea, one Earth's most active volcanoes, is ever-changing, but often you can stand on recently cooled lava and see red through the cracks and watch melted rock rivers spin and hiss into the ocean. Visit the Volcano House, Kilauea Iki Crater, and Jagger Museum.
Diving and Snorkeling
The Kona Coast has the clearest water and the most accessible reefs and lava tubes, with tremendous varieties of fish (huge manta rays included), dolphins and sharks (try diving near the Captain Cook Memorial), but Hilo's harbor also abounds with underwater sights, and everyone should see the huge sea turtles at the Punalu'u Black Sand Beach on the island's east side.
Surfing
The shores of the Kona and Hilo sides of the island both have surfing beaches, with rentals and instruction for surfers of all experience levels.
Cruises and Whale Watching
From the comfort of catamarans or glass-bottom boats, you can see and learn about humpback whales migrating through the islands, dolphins and swarms of reef-fish, either as part of large tours or private charters.
Waipio Valley
Up north in the Hamakua Coast area between the Hilo and Kona sides, where Hawaiian monarchs once hung out, hike or ride horses into this regal valley, with its waterfalls and black sand beaches.
Rainbow Falls State Park
Near Hilo, take in the falls' namesake rainbows at sunrise as the 80-foot drop in the Wailuku River crashes beside you, or catch its biggest flow on a rainy day as you enjoy the grottos of lush vegetation. The stairstep-like Boiling Pot pools are nearby on the same road.
Nude Beaches
Kehena Beach (Dolphin Beach) a favorite of Bohemian locals is a mediocre beach off Highway 137 on the Hilo side. Honokohau Harbor Beach in Kaloko-Honokohau National Historical Park on the Kona coast is a gay nude beach. The owners of Steam Vents beach, near Hilo, with its cave-hidden steam baths, allow its use by the public.
