The single most popular activity in the park is orangutan trekking out of Bukit Lawang, where a long-running rehabilitation programme has produced what is probably the most accessible population of Sumatran orangutans anywhere in their range. Treks run from a few hours to several days and almost always include sightings of gibbons, leaf monkeys and macaques as well.
For deeper forest, head west to Ketambe on the Aceh side. Trips from here climb hard into the interior and offer better chances of finding the rarer wildlife — including the parasitic Rafflesia flower, the Sumatran tiger (in tracks rather than person), and the occasional elephant. Serious mountaineers can attempt the summit of Mount Leuser itself from Kedah — a two-week expedition through some of the wettest forest on earth.
The Bohorok River at Bukit Lawang gives tubing and swimming; the Alas River is one of the great rafting rivers of Southeast Asia, running multi-day trips out of Ketambe. For a quieter alternative to Bukit Lawang, the village of Tangkahan has a small herd of working elephants used in park patrols and welcomes visitors to the morning river bath.