Gunung (Mount) Sibayak is located outside the town of Berastagi in North Sumatra. It is the most accessible of Indonesia's active volcanoes and can be hiked in half a day. The classic approach is to catch a bemo, or public minibus from the town of Berastagi to the villages lying at the foot of the mountain, and walk up the steep, paved road to just below the top of the mountain. This ends in what amounts to a dirt parking lot with a large drainage pond, and it takes some looking to find the trail that continues to run up to the mountain top. This is where the first sign of an active volcano can be seen: a scalding hot water stream with greenish, sulfurous water. The trail comes out in the volcano's crater, which is more than 7,200 feet high and firmly among the clouds. Sulfur-spewing fumaroles are everywhere. Scouting the crater edge will reveal the path down the other side of the mountain, in this case through a thick jungle. This path ends at a pair of basic spas that take advantage of the natural hot springs on this side of the mountain, so remember to bring a pair of swimming trunks.
Gunung Bromo, located in East Java, is a classic stop for those making their way between Yogyakarta and Bali. It is actually the crater of a single super-massive volcano of ancient origin, and several smaller volcanoes grew up inside it. One, Bromo, is still active, and hikers can go right up to Bromo's crater edge and even walk around it. The highest of these mini-mountains is Gunung Penanjakan at 9,088 feet, and it makes for a great place to watch the sun rise or set. This is also a volcano for day trekking, and the walk from the caldera rim to Bromo itself will take only about a half-hour. However, the greater caldera is a vast landscape more akin to Mars than Earth, and it is worthy of several hours of exploration. Furthermore, the outer slopes of the great caldera are packed with farms tilling the rich soil of the steep slopes, and anyone looking for a strenuous challenge can go on an exploration hike there.
Gunung Agung and Gunung Baktur
At 10,308 feet, mighty Agung is the tallest mountain in Bali and one of the tallest in Southeast Asia. It last erupted in 1963 but shows few elements of active vulcanism. However, it makes for a good trek. Trekkers in a hurry can scale Agung in one long, hard day, but it is encouraged to spend the night at the top and watch the sunrise before coming down. The standard Agung guided tour is typically advertised as a 2- to 3-day trek. Also on Bali is Gunung Batur. While smaller at 5,633 feet, Gunung Batur has more volcanic attractions, such as fumaroles and hot spots, and can be easily hiked in a single day. Either can be reached from anywhere in Bali.
Eclipsing Agung is Gunung Rinjani, a 12,224-foot titan that erupted in May 2009. This mountain is located on Lombok, an island neighboring Bali. The typical trek is a 3-day affair, and the volcano features hot spots, hot springs, a crater lake and fumaroles. It is strongly encouraged to at least hire a local porter, as the people living around Rinjani can become violently angry at trekkers who do not employ locals as porters or guides.
At 9,738 feet, Gunung Merapi is famous not for its height, but for its fire. Located in central Java near Yogyakarta, it is the most active volcano in the world's most volcanically active country. The mountain is always smoking and usually rumbling on top of that. Its bare, deforested slope is testament to its frequent eruptions, and charting a path to the top usually requires a local guide, as these paths can change following eruptions and landslides. Also, planning a trek up Merapi is a hit-or-miss proposition, as there is a fair chance the mountain may be too dangerous to approach. However, the hike to the crater rim can usually be done in one long day.