The Kalalau Trail is a backpacking trail along Kauai's Na Pali coast. Stretching 11 miles one way in to a beach camping spot on one of the world's most beautiful coastlines, it's a very rugged and hazardous route. It's also volume restricted for most of its length, requiring a permit and reservations to hike and camp.
The first two miles of the trail are openly accessible as a day hike, however, ending at Hanakapi`ai beach. There's another 2 mile spur heading inland and uphill to Hanakapi'ai Falls.
Fewer people do the extra 2 mile spur up the the falls. While it's only 2 miles, it's very dense and very muddy.
And suuuuuper fun.
Wear water shoes (something like these), because there are five stream crossings. While we watched other people stop and try to figure out a way to cross without getting their hiking boots wet, we just strolled right into the water. It makes the crossings easier and more fun, and you get to cool your feet off frequently, which is a very welcome bonus in the heat and humidity.