Well of course, one thing you would have troubles missing in Java, Indonesia is volcanoes.
Pretty much everywhere you go, you stumble on them at some point.
I would definitely recommand to climb the unmissable Mount Bromo wich is famous world wide. But don’t miss the impressive Kawah Ijen where you are going to be astonished by the workers carrying bare hands huge and heavy loads of sulphur across a small road.
The one you see on this picture is Mount Merapi (Gunung Merapi or if you prefer Moutain of Fire) and is 45 km from Yogyakarta.
Still young and active, it is highly monitored since 2006 for fear of an eruption.
I had to crop the picture to get this japanese feeling wich is enhanced by this beautiful blue.