I will ask DH. He grew up in Indonesia and has traveled fairly extensively. I have never been to Sumatra- just Java (where IL's live), Bali, and then further east by Flores when we went to see the Komodos.
I know nothing about Sumatra; I just wanted to chime in and say that I'm jealous that you are on that side of the world and able to do all of these fun trips so easily. I want to go back!