Go kayaking (or canoeing) at night in the bioluminescent bay in Puerto Rico. It literally glows at night. go off the main island, if you go out of the bay it's a beach that you can only get to by kayak or swimming.
Jacob's Well -- Wimberly, Tx: One of the most significant natural geologic treasures in the Texas Hill Country. It is one of the longest underwater caves in Texas and an artesian spring. Jacob’s Well surges up thousands of gallons of water per minute!