Borichinda Cave sits in the forested hills southwest of Chiang Mai, accessible via a roughly two-hour hike from Mae Klang Waterfall in Doi Inthanon National Park. The cave itself is modest -- stalactites and a few chambers -- but the walk through dense tropical forest is the real draw. Bring a flashlight and sturdy shoes; the trail can be slippery in the wet season.
The area around Mae Klang Waterfall makes a good picnic spot before or after the hike. A day trip from Chiang Mai combining the cave, the waterfall, and the national park works well if you have your own transport.