The National Park of Mount Leuser, one of our planet's last jewels of biodiversity, is best explored from April to September. A UNESCO site, its lush tropical forest is home to some of the rarest plants and animals in the world, among which the last great mammals of Sumatra: elephants, tigers, rhinoceros and orang-utans. The village of Bukit Lawang and the hamlet of Tangkahan, from where you can explore the jungle on an elephant's back, are on the doorstep of the park.