Wat Umong (or Oomong) is one of my favorite temples in Chiang Mai. It can be an especially magical place just after the rainy season has ended, around early October or November.
Wat Umong is unique in many respects. For one, it's built in the foothills of Suthep mountain and is still heavily forested. But the most unique feature of the temple are the tunnels which give the wat its name (umong is the Thai word for "tunnel").