Doi Luang Chiang Dao is the third-highest mountain in the country and the story is that it is hollow, with a city inside it. The lord Kham Daeng went in after a golden deer and stayed, and is the guardian spirit of the whole north from in there.
Five caves run back into the limestone. Two are lit and walkable. The rest need a woman with a pressure lamp, hired at the mouth, and she is the reason to go.
A full day with a driver, and the only stop on this route outside the valley.
The shrine at the cave mouth is Kham Daeng's. A khan tang, before you go in rather than after.