About 6.5km northeast of Muang Saen, Wat Phu Khao Kaew was built on the site of some presumed pre-Khmer ruins, making it a holy spot for locals. Nothing of that era is visible; now there is a bright red and gold modern stupa and a large reclining Buddha in the arm-down, 'Entering Nirvana' posture. It sits atop some exposed bedrock and the beautiful Mekong-side perch is more of a reason to stop than any of the structures.
It is also believed to be home to a naga (river serpent), though the entrance to its lair is covered. Don't be confused by the sign on the road that misspelt the last word as 'Kae'.