Easiest way to get around is by becak pronounced bay-chuk. It's worth the experience. You can find these old bikes only in Siantar. Old BSA made in England. Special sound, special look and every becak is unique and lovely modified. Cost about 20.000Rp to go 3km.
The cheap way is to go by minibus, which cost only 3000Rp for same distance, but without help you will not find your way. There are no bus schedules.
The only place I know to rent a motorbike is the Siantar Guesthouse. You can rent 150cc Classic Vespa for about 60.000Rp per day.