Nam Tok Soi Sawan is a 25m-tall waterfall flowing from June to December. It's a 19km drive from the visitors centre and then a 500m walk, or you can hike with a ranger (this must be arranged in advance) for about 15 largely shadeless kilometres along the top of the cliff.
What the park calls Thailand's largest flower field blooms from October to January (November and December are best) next to the falls.