Maolin Valley (茂林國家風景區) is one of the most beautiful places in Kaohsiung and Pingtung counties. It's about a two hour drive from Kaohsiung City and has many scenic locations on the way as well.
On our trip we decided to drive our scooters. The first hour we pretty much just drove north towards Tainan then started to head east. I read on another blog about mud volcanoes (燕巢烏山頂泥火山) but it wasn't clearly marked. Once you get onto the mountain there are signs marking the way.
There were some mud volcanoes that you could easily look down into and watch as the mud bubbled up and splashed over the rim. The mud wasn't hot at all; we didn't do this, but apparently you're able to light the mud volcanoes on fire.
Mud Volcano Wiki