"Kamar Ganti" means "Changing Room" in Indonesian.
This small changing room is built to accommodate people who wish to take a dip in the pool at the bottom of the hillside waterfall that travelers encounter on the way to Bukittinggi (a tourism township located in the highlands of the Province of West Sumatra).
The notice board on the door says "do not urinate in here except 4-legs (dogs)"
Also, the shorts seem to be for public use. I dared not touch them...