In Reply to: Not sure which part or how long... posted by questionsquestions on Friday, 13. June 2014 at 13:19 Bali Time:
and not sure they are direct from Surabaya either
If only 7 days then I'd choose which part of Sumatra you would prefer eg west, south, north
If 7 days includes travel time to get there then it's not all that long to explore.
For example if you choose North Sumatra and wish to see Lake Toba and surrounds, you will need transport and travel time from Medan to outlying areas.
Roads to some areas/villages are definitely not great to travel on