Nam Tok Phaeng is protected by a national park; this waterfall is a pleasant reward after a short, but rough, hike. After the waterfall (dry out of season), it's a further exhilarating 15-minute climb up a root-choked path (along the Phaeng-Domsila Nature Trail) to the fantastic Domsila Viewpoint, with ranging views. The two to three hour trail then continues on through the jungle in a loop, past other waterfalls before bringing you back. Take water and good shoes.
It's possible to continue up to Khao Ra, the highest mountain on the island at 625m. Those with eagle-eyes will spot wild crocodiles, monkeys, snakes, deer and boar along the way, and the viewpoint from the top is spectacular – on a clear day you can see Ko Tao. Although the trek isn’t arduous, it is very easy to lose one’s way, so consider finding a guide.