Surfing
The Indonesian archipelago is one of the world’s top surfing destinations. The best time to surf is from April to September with the best waves generally found on islands facing south and southwest, including Bali, Flores, Java, Lombok, Sumatra, Sumba and Sumbawa. Some well-known surfing beaches, such as Ulu Watu on Bali, tend to get overcrowded, but organized trips to isolated areas are widely available. Surf camps such as those at Cempi Bay (Sumbawa) or Lagundri Bay (Nias) offer basic accommodation and simple food. Windsurfing is particularly popular on Bintan Island and Sanur and Nusa Dua beaches on Bali.

Diving
There are approximately 80,000km (50,000 miles) of coastline, reputed to contain 15 per cent of the world’s coral reefs. In spite of the obvious opportunities, Indonesia’s diving industry is still relatively young, though the number of companies offering courses and excursions is rising rapidly.
On Java island, the best diving is on the west coast, where three volcanic islands mark the remains of the Krakatoa volcano (which last erupted in 1883). Bali’s tourist stronghold in the Kuta, Nusa Dua and Suar triangle offers easy and moderate diving, with easily accessible reefs. Tours to more remote (and less busy) areas are available. On the northern tip of Sulawesi island, the Taman Nasional Laut Bunaken Manado Tua is a national marine reserve with particularly steep coral walls; international air connections to the island facilitate access. Further north, the lesser-known Sanggihe-Talaud and Togian islands are reached by live-aboard dive boats. In the south, Take Bone Rate is the world’s third-largest atoll, while the Tukang Besi islands have featured extensively in the films by the French underwater explorer Jacques Cousteau. Nusa Tenggara’s most popular sites are the three Gili islands near Lombok, whose calm shallow waters are ideal for beginners. Maluku consists of approximately 1000 islands and has only recently been discovered as a top diving destination. Southeast of Ambon, the Bandana islands are accessible by air and offer a number of sites suitable for beginners and experienced divers. The major resort in the Sumatra and Riau islands is Bintan, easily accessible from Singapore. The clearest and most colorful dive sites are in Pulau Sikuai off the Padang coast (western Sumatra) and Pulau Weh off Banda Aceh (northern Sumatra). Irian Jaya also offers good diving around the famous Mapia Atoll (where dolphins and killer whales can sometimes be spotted) and the waters of Cenderawasih Bay off the western end of Bird’s Head peninsula.

Climbing
The island of Sumatra is perhaps the best location. Gunung Padang, near the island’s capital, Padang, is a small black basalt cliff reached via a river-paddling trip followed by a trek through rainforest. Further inland, Bukittinggi offers challenging cliffs overlooking rice paddies. The dramatic canyon rocks in nearby Harau National Park are still largely unexplored and should only be attempted by experienced climbers.

Caving
Indonesia’s most accessible caves are on the island of Java and include Luweng Jaran, stretching over 20km (125 miles) beneath the Gunung Seuw mountain range; Gua Barat, which has the longest underground river system in the southern hemisphere; and Gombong, whose stone towers rise spectacularly to some 40m (132ft) above sea level. On Kalimantan island, Mangkalihat offers a rarely visited underground world of giant limestone corridors. Even less explored are the isolated caves near Wamena on the remote Bird’s Head peninsula in Irian Jaya.

Sailing
The majority of companies offering whitewater rafting are located in Bali, where several rivers – including the Ayung, Balian, Telega, Ubaya and Unda – are commercially rafted. Spectacular rapids can also be found on the Citartik River (western Java), the Sadan River (Sulawesi) and the Alas River (Sumatra). Rapids are generally at their strongest between November and March. River tours up the great Mahakam River on the island of Kalimantan, which is dissected by a network of rivers running from the mountainous interior to the coasts, are billed as a trip into the ‘heart of darkness’. Starting from the port city Samarinda, such tours last for several days (with onboard accommodation available) and continue deep into the upper jungle reaches, where tribal communities have largely preserved their traditions.

Trekking
Indonesia has some 120 active volcanoes and numerous volcano treks are possible: on Java island, popular volcanic destinations include Krakatoa (reached by a five-hour boat trip followed by a 30-minute climb), Mount Bromo (the most visited of Indonesia’s volcanoes) and Kawah Ijen (whose crater is filled by a turquoise-blue lake). Those preferring dormant volcanoes may head to Gunung Agung in Bali (known as the ‘Navel of the World’), Gunung Rinjani on Lombok island (which has hot springs at the top and is revered for its mystical qualities) and Keli Mutu on Nusa Tenggara Barat (whose crater contains three spectacular mineral lakes). For jungle trekking through the Indonesian rainforest, the islands of Irian Jaya, Kalimantan and Sumatra offer the most remote and untouched terrain. The best trails include trips to Bukit Barisan National Park, a remote and beautiful peninsula in Sumatra (with routes leading through tropical rainforest onto a beach inhabited by turtles); the Muller Mountain on Kalimantan (with a trail following the traditional jungle route used by the native Iban people); and Lake Habbema on Irian Jaya (a week-long trek to remote villages and mountains).

Ecotourism
Having been criticized in the past for the destruction of large areas of its rainforest through forest exploitation, the Indonesian government is now keen to encourage an environmentally friendly tourism policy. The growing trend for back-to-nature holidays means that numerous types of eco-tours are available. In the Tukangbeshi archipelago near Sulawesi, tourists have the opportunity to participate in coral reef preservation projects by helping to collect scientific data.