Trekking out of Bukit Lawang to find orangutans is the single most popular activity in the entire national park, and for good reason: nowhere else on earth makes seeing these animals so straightforward. Half-day, full-day and two- or three-day treks are all on offer; the longer trips camp in the forest and have a much better chance of encountering shy, fully wild animals as well as the more habituated ex-rehabilitation orangutans. Gibbons, Thomas leaf monkeys and macaques are reliable companions on any trek.
All trekking is guided — that is a non-negotiable park rule, and a sensible one. Reputable operators in Bukit Lawang are members of the local guiding cooperative and follow agreed rules about distance, feeding and group size. Avoid any guide who promises a guaranteed sighting or who offers to bring food to attract animals. Pay properly: rates are set locally and the higher prices fund park fees, conservation contributions and the guides' livelihoods.