This yearly festival is a true favorite with our family. Water, food, music, games, open space... it's hard to find anything better to do over summer.
I've posted detailed information about this over the years, but this year organizers have managed to get a fairly complete English version of the site together in time. (It's funny to read my own post from a couple of years ago, and to see how things have improved.)
The festival is based on the banks of the Dong Shan River in Ilan. There are shuttle buses going there on a regular basis during the festival. Be warned, weekends are insanely busy.
You can buy entrance tickets in advance if you want to save a little money.
Full information can be found on the English website.