PingLai Waterfall in the hills behind Xinyi in Nantou County is quite spectacular after rain but can be bone dry at other times. Read up on it here: http://taiwandiscovery.wordpress.com/20 ... ai-stream/
You would be wise to make enquiries with Jade Mtn N.P. HQ or TungPu cop shop about the permit situation. Lots of people just going up to the falls don't bother, but at CNY they sometimes station a cop car at the trailhead. You also might want to enquire about the state of the waterfalls. Like PingLai, they are often reduced to a trickle in the dry season.There are only a few spots on the way to Patungkuan with camping possibilities, usually the old Japanese police sites, so if you go don't camp elsewhere because large sections of the path are directly under cliffs.
I can't remember if Aowanda has any waterfalls but it's quite spread out with several minor paths in the 'tourist containment area'. The big hiking possibility is at the back: over a footbridge and then following a path up a large river valley. The path goes up to the Central Mountain ridgeline.
Nengao at CNY depends totally on the weather. Quite a lot of people get rained out and there is the danger of mudslides in the damaged parts of the path. I wouldn't go here unless the forecast is really good and that means taking note of the Hualian County weather forecast as well as in Nantou County.
There is a good waterfall just off the Provincial Road #21 up from NanChuang about 2kms before Luchang in Miaoli County.