you can do east to west tour starting at Jakarta and taking a train to Bandung, and then onwards to Jogja and then Bromo and take a boat across to Bali and then to Lombok.... looking at 5 volcanoes ..... a week and a half imho at least
Krakatau is magnificent but you will need to be on a boat/ship to get there and its located between Java and Sumatra .... lots of ferries ply this route between west java and Lampung
Suggest you start Jakarta and get to Bandung to cover Gede and Papandayan and continue eastwards to Jogjakarta to cover Merapi..... and continue eastwards to cover Bromo ..... and continue eastwards and cross over to Bali to cover Agung ...and continue eastwards to Lombok to cover Rinjani
In Irian Jaya, there is a volcano with snow on top, the name is Jayawijaya..
If u want to go anak Krakatau, i think there's some daily trip from Carita to go there.
U can googling for it.
In Bandung, there's 1 volcano that still natural and really quite.
only few persons know it, the name is "kawah putih" or if u want to go to the "more popular one ", u can go to "tangkuban perahu".
it's nearby to Bandung.