There are endless trekking opportunities in Indonesia. The most popular volcano treks include Batur on Bali and Bromo on Java; more taxing favourites include Gunung Rinjani on Lombok, Kerinci on Sumatra and Semeru on Java. In Sumatra, the Gunung Leuser national park is Southeast Asia's largest, and includes the famous Bukit Lawang orang-utan sanctuary. Many routes need guides , and not just to find the paths: turning up at a remote village unannounced can cause trouble, as people may mistrust outsiders, let alone Westerners. Guides are always available from local vilages and tourist centres.