Originally Posted by
A380 Flyer
Ideally I would love to fly to Medan and do Bukit Lawang, Lake Toba, and the Harau Valley at Bukittinggi, but getting from there over to Jakarta is too much hassle seeing as there is little train service in the whole island (just Palembang to Bandar Lampung in the whole of Sumatra?).
Not quite sure what you mean by "too much hassle". There are probably tens of flights daily between Medan and Jakarta, so the actual flight will be no problem at all. However, Sumatra is large and undeveloped even by Indonesian standards, and the trains there are creaking relics that make the Javan train system look shiny and new. Your itinerary sounds pretty aggressive as it is, so I'd probably stick with Java and Bali if I were you, and leave plenty of time for Bali as it's the bit of the archipelago that most visitors (including yours truly) find the most pleasant.
But if you do decide to go for the train, Seat 61 may prove helpful:
http://www.seat61.com/Indonesia.htm