Although it's not the largest waterfall on the islands, Khon Pa Soi Falls is still pretty impressive, and it never gets crowded due to its isolated location. From the little restaurant (sometimes only serving cold drinks) cross the big, fun (or scary, depending on the person) wooden suspension bridge to Don Pa Soi island and follow the roar 200m to the main waterfall.
You can find bathing spots when the river is low, but be seriously careful here and don't get caught in the current.
A second smaller bridge takes you to some of the old logging diversion walls the French built in the Mekong for their logging operations.
