Koh Phangan, Thailand
You can't walk anywhere on the island of Koh Phangan without seeing other 20-somethings just about everywhere. Credit the crystal-clear waters, the boundless bars, or the really cheap cost of visiting — all are reasons to attract a fun-loving crowd. Plus, the island is home to the infamous Full Moon party, a night (and morning) of dancing on the beach that draws young people from all over the world.
Source: Flickr user oksidor