Gunung Leuser National Park is a rich rainforest, and its most beloved inhabitant is the orangutan. It’s home to the Sumatran orangutan, which is slightly different to the Bornean orangutan. These primates are regularly spotted in the wild here, though logging is a bit threat to these animals.
It’s worth visiting the Bohorok Rehabilitation Centre, a rescue centre for orangutans in Bukit Lawang. You could meet these animals when you go for a big walk in the jungle.
More information: http://www.indonesia.travel/bukit-lawang