17 KM from the Hotel / Casino on Bokor Mountain is the Popokvil Waterfall.
The falls aren't impressive unless it's raining (and you don't want to go out to see them if it is), but the park and area is very nice.
Climb out on the rocks, across the streams, sit in the water (a bit cold), and have fun for an hour.
There's a 50 entrance fee for adults, but you get a bottle of Bokor Water for the price.
A large, airplane hangar like restaurant is just outside the waterfall, with indoor and outdoor seatin, and Khmer food and a buffet during the day.
The falls are almost on top of the mountain, so usually there's no much water. Which makes it much better to explore around the area.