Hallo ! One month is a sufficient time , and just nice. me and frineds from europe did the route like this : we started the trip from medan ( which is the capital of the northern part of sumatra ), we stayed in bukit lawang by the riverside accomodation for 02 nights, then straight to lake toba stay 02 nights, padang sidempuan ( sipirok 1 night transit ) bukit tinggi 2 nights, lubuk linggau 01 night, lampung 01 night, bogor 01 night, jogyakarta 02 nights, bromo volcanoe 01 night, kawah ijen 01 night, and the rest we spent the time in bali island ; snorkling in menjangan island, tulamben shipwreck for snorkling, and so on we did it for 32 days, and it was fantastic we had this trip by having a car with a reliable driver and a guide for a reasonable quotation . and I guess thats the best way to do when you wish to see the interior of indonesia, especially when you have a limited time, perhaps you could send me a message and I will be able to pass you the contact number of this guy wo did everything for us. no regret , indonesia such a nice country.
cheers
