Once you get past the paved waterfall trails in this scenic park 20km southeast of Sansia, you reach natural hikes that climb for hours through sweet-smelling cedar forests. Public transport to the park is limited; from Yingge train station, it's around a NT$500 taxi ride.
The main trail starts up a short incline to the right of the toilets at the end of the paved route to Manyueyuan Waterfall. There are many side branches but the main route connects Manyueyuan with Dongyanshan Forest Recreation Area. However, there is no public transport to and from Dongyanshan, so if you walk there you must walk back. It's about a four-hour hike one-way.
Two very worthy diversions on the main trail are up to Beichatianshan (北插天山; North Sky-Piercing Mountain; elevation 1727m), the highest peak in the north, and going further afield to a stand of giant ancient cedars (神木; shénmù).
To get to Beichatianshan, take the main trail to its highest point and then turn left, following the English signs. It's a long day hike to the summit and back (expect 10 to 12 hours), so many people make it an overnight trip. There's a wild campsite near the base, beside a rushing stream.
The trail down to the old cedars follows the same path as to the base of Beichatianshan and then drops down a side trail, but this is not clearly marked in English. Get a map, or follow another hiking group.
You can take bus 807 from Sansia Bus Station to the Manyueyuan Forest parking lot, but it's infrequent (every 1 hour 40 minutes).