About 17 kilometers (10 miles) from Mae Hong Son town is the small Fish Cave (tam plaa in Thai) national park. The name is a bit misleading. There's no cave that you can really go into. The park surrounds a place where a small stream emerges from the rock face of a hill.
However, there are definitely fish. Big brook carp of a kind found only in a very few places in Thailand. The fish grow up to a meter (three feet) long and appear deep blue in color. The fish seem to like to cool water of the cave, and you can see them 'queuing up' to get in. There's another small opening a few steps up that allows you to look down into the cavern, which appears to be filled with the fish.
Photos of the area around the fish cave. Click on any of the thumbnails to see the full size image.
The park is a cool shady place with forests of trees lining the cool stream flowing out of the hill. Beyond the stream, near the park entrance, is a large open space suitable for pitching a tent. There are also a number of food stalls and restaurants around the entrance.
While the Fish Cave itself is not a really spectacular sight, the cool environment and ample amenities make it a very pleasant spot to plan a lazy afternoon.
Admission to the Fish Cave is 100 Baht for foreigners.