Yaming Mountain is a breath of fresh air and only a quick trip away from the crowded streets of Taipei. It's a great place to go to see a little greenery and breath more freely.
Yamingshan Park (at the end of the bus route) is a nice area to begin exploring. There are flowers, ponds, hiking trails, and waterfalls in this area. It's really enjoyable, but can get a little old after a few visits.
Other parts of the mountain are also worth exploring. On the parts of the mountain that are not part of the national park, there are many small coffeeshops or restaurants, which are fun places to relax. Some even offer outdoor seating or views of the leaves changing color in the fall.
Inside the national park are also many other areas to explore. There are a variety of hiking trails, recreation areas, waterfalls, and fumeroles to explore.
Quick Tips:
The best source of information that I've found about Yaming Mountain actually came from the Taipei City MRT maps! There is one map they publish that contains the MRT map on one side and information about local sights on the other (all in English). Under the "natural sights" on that map were several Yaming Mountain areas described. Pick up a copy of the map in English and Chinese, and you'll have more luck asking for help if you get lost (the maps are identical, just printed in different languages, so find what you're looking for on the English one, find the same place on the Chinese map, and point!).
Best Way To Get Around:
There is a bus up to the mountain, but once you get up there, it's hard to get around. On the weekends and holidays there is another shuttle bus that will take you around, but I've never used it (I try to go on the weekdays to avoid the crowds). To really get out into the park, I prefer to ride my scooter up the mountain (although coming down is a little scary if there's lots of traffic!!).