Suoi Tranh waterfall is around 7km east of Duong Dong. Once you've paid the small entry fee it's a 10 – 15 minute walk along a well-trodden path through the jungle. There are many rock pools in which to take a dip, at the top there's a 4m waterfall (sometimes from January to May water can be a little lacking), the whole place has a lovely ambience.
If you want to see a more spectacular waterfall then Da Ngon is what you're looking for, the pool at the bottom is famed as a 'natural Jacuzzi'. It's located deep in the jungle so a guided tour is the best way to see it, tours take half a day. Suoi Da Ban (stoney surface spring) is a natural spring that comes up through granite rocks. The white water cascades down and some of the pools are deep enough for a swim. It's easily accessible from the road.