I am a big fan of Sumatra, but I wouldn't recommend Medan as a spot to stay for more than a couple of days. The Muslim areas of Indonesia tend to be the friendliest to foreigners- in my opinion- but Medan is an exception to that rule. Sumatra lost a LOT of tourism after the tsunami ten years ago and hasn't quite recovered. There is an abundance of guesthouses and tour guides, but very few tourists meaning there is a lot of attention on you.
If you can handle not great internet speeds, I would spend most of my time in chill Lake Toba and make my way up to Pulau Weh (passing through Bukit Lawang for orangutan trekking- truly incredible, check out Thomas' treks- and Banda Aceh for the tsunami museum). Spend the rest of your time chilling and diving on Pulau Weh.
Let me know if you have any other questions about Sumatra. I loved it, but Medan wasn't what I was looking for. There are far friendlier cities for that big, not too developed, bustling city feel if that is what you are craving!