Tham Lot (aka Tham Lod) cave is one of the largest and most beautiful caves in Northern Thailand, not very far from Pai. Definitely worth a trip. The cave is visited with bamboo rafts that guides will stir down the shallow river that flows in the cave.
At several points the boat stops and you can visit large side chambers with cave formations (speleothems). Thousands of birds and bats live in the cave and you can see them at dusk and dawn.