Mount Merbabu is one of the most popular hikes in Central Java, and has spectacular views to other volcanoes in all directions from the summit area. It has five calderas, Condrodimuko, Kombang, Kendang, Rebab and Sambernyowo but no serious volcanic activity has been reported here for many decades.
Merbabu can be climbed from Kopeng (northern slope, near to Semerang) or Selo (southern slope between Merbabu and Merapi and closer to Solo),
The best hike is definitely a traverse from Kopeng to Selo, because the climb from Kopeng is a gentler slope and there is a good source of water halfway up. This hike could be completed in a day by very fit people, but an overnight stay on one of the summits is recommended.