Berastagi is a couple of hours in a minibus from Medan, sit at the back if you don’t like overtaking round blind corners. This is a city in the Barisan region, and popular because of the two volcanoes and hot springs. It’s likely that your accommodation will offer to arrange trips, or you can go in to town and use the “tourist information” near the roundabout, or one of the others. I booked a guided trip to Sibayak with my hotel which was a fair bit overpriced at 400,000 per person, and after decided I’d book nothing else with them and source my own transport. The volcano is definitely worth doing, I think it’s walkable on your own, though you could be put off by the deaths list adorning the wall of the tourist office.
The food in the town is good, and the fruit market is excellent, but haggle for prices if you can. The food market at is at night on the weekends when more people arrive from Medan. Satay chicken, roast pork (Babi pangang) (pork is sometimes called B2 because of the two b’s in the name), and if you’re inclined, dog (Biang – known as B1 because of the single b). I didn’t try the dog. I think this is a tough place to be a vegetarian while eating out, you could stick to fruit and raw vegetable, but I get the impression that most veg dishes are cooked in pork stock.
One annoyance with my tour was not seeing one of the ghost villages at the foot of Sinabung, despite discussing this, it was requested we pay more for additional bits in the afternoon. Personally I get annoyed by bullshit, I know some people say it’s part of travel but once someone’s given me a load, they are losing my further business and tip money. So on my trip onwards to the south, I arranged for a driver to stop at a ghost town for photos. The lesson is to haggle, and get a tour provider to write every details down on your receipt and state whether it includes entrance fees or not.