This very active volcano had a large eruption in 2010. Post 2010 the mountain became significantly more difficult to climb in fact the true summit is no longer reachable without ropes, so trekkers have to be content with the view from the rim about 20 m below the summit, which is very impressive. The hike can be done in a day and there are a number of guesthouses in Selo.
Do It Yourself Information
The hike to the plateau below the summit and crater rim is straight forward follow the big wide path that starts to the left of the "New Selo," sign above town. There are paths that go off to fields on the left and right but just keep taking the path that goes straight and up. In about 2 hours walking time you will get to a large rocky plateau. There is small peak here (with a new plaque in Indonesian which appears to commemorate someone who died on 18th of September 2011) before you descend a small ways to cross the plateau. This is the best place to catch the sunrise rather than trying to climb the cone in the dark like I did. I got stuck (ran into a cliff) had to wait for dawn to see the way and traverse around to rejoin the "trail" up to the crater. The way to the crater follows a ravine just to the left of the central peak as you look at the mountain from the plateau. It is a steep with loose sand and rock. Beware of rocks falling form other climbers. And if you do knock one lose yourself give a "Hati Hati" (beware) shout to those below. The new crater has very shear cliffs and is quite impressive. But watch your step one slip leads to certain death. At the crater there is a small safer path that lead to a vantage point to the left. A bit of a more "adventurous" path leads up to the right to what may or may not be the true summit. The ridge further to the right may be higher it was difficult to tell and this was definitely a no go to reach. If your confident in hiking do it yourself, if not you may want to consider a guide.
Selo can be reached by bus from Solo or from Yogyakarta