The Northeast coast of Kauai, Hawaii, aka the Napali Coast, is regarded by many as one of the wonders of the world.
The road that encircles most of Kauai doesn't make it all the way around, simply because the cliffs and mountains of the Napali Coast are too intense to blaze a road through. The only way to see the coast is to travel alongside it in a boat or raft. In fact, many groups offer Zodiac or raft tours that include journeys inland to ride ziplines, explore sea caves, and bounce around in the surf.
In July 2010, my family spent 12 days in Kauai, and had an absolutely mind-blowing time. Did a full blog write-up, and posted a ton of images (slideshow below). Many adventures, but our Zodiac tour along the Napali coast was definitely a highlight of the trip. Dying to do this again!