Within the protected Peradayan Forest Reserve, this recreational park offers the chance of a 2km (one way) hike to the top of Bukit Patoi (310m). The well-marked trail, through pristine jungle, begins at the picnic tables and toilet block at the park entrance, about 15km southeast of Bangar (towards Lawas). It's best organised through a BSB tour company. To get here from Bangar ask around for an unofficial taxi (about B$30 return). Bring plenty of water.
Once you reach the peak, enjoy the views then turn around and come back down – in theory it is possible to continue on to Bukit Peradayan (410m), but the path is poorly maintained. Up-to-date trail information is available from tour companies in BSB.
If you want to explore the Bruneian rainforest without the logistics and expense of a trip further upriver, Peradayan Forest Reserve makes a good alternative that can easily be done as a day-trip from BSB.