Qingshui is a small farm town near Taichong, the second largest city in Taiwan. During my last visit, the rice paddies looked in need of some rain, but the longyan trees were plentiful.
They taste like lychees, but the skin is much thinner, there is less meat and the pit is beautiful, smooth and black. If you save enough you can use them as toys and invent your own game.
If you go to Taiwan, hopefully you can find a friend who can bring you to a farm to eat them, but otherwise, you can find these in markets and even the Chinatown streets of NYC.