It's amazing to think how close to Taipei you can find a volcano crater with activity!!! It is not that it is active, but it does continuously expel water vapor. You will see the special smell that there is when approaching! To see it, if you go by car or motorcycle, you can stop at Xiaoyoukeng Bridge (there is an area enabled for it), on the way to Erziping. From there it looks great.
If you go by public transport, the 108 bus stops at the Xiaoyoukeng Recreation Area, where you can see it more closely even if the view is not as beautiful as on the previous bridge.