Sumatra and Borneo are the only places in the world to see wild orangutans. The best place in Sumatra is probably Bukit Lawang in the rainforest at the edge of Gunung Leuser National Park. The rehabilitation centre is placed up river on the jungle side from the village. In good Indonesian easy-going-style, nothing is fenced off at the feeding platform, so you will get VERY close to the orangutans, so keep your distance to the big ones. It's also possible to do different treks through the jungle with a local guide to see the same orangutans, and maybe some others, in their natural habitat.