The area around Berastagi is the main fruit, vegetable and flower growing area for North Sumatera, and is famous for Marquisa, or passion fruit, which is a popular drink and which is grown only in a few part of Indonesia. Berastagi's elevation of 4,600 feet above sea level gives it a cool climate, so it is a comfortable place to stay.
There is a golf course walking distance from the town centre, and just ten minutes from 'Marquisa town' by bus (Rp 500) is Tahura National Park, a beautiful spot for exploring the jungle. There is a good waterfall and many clear and easy paths. You will almost certainly see wildlife a short walk into the jungle, gibbons or other primates almost certainly, and a good chance of seeing a hombill or some other exotic bird.
The park also has elephants and horses if you fancy an hours ride. It can be very pleasant just riding around Gundaling Hill for the views and the cool air. Admission to the park is Rp 1.000. There is more to do around Berastagi than just climbing volcanoes, it is also an excellent centre for jungle trekking. Its also only a Rp 500 bus ride away from Sibolangit Botanical Garden - a circular path around the perimeter and many paths within. There is wildlife apart from huge trees and exotic flowers. Wear appropriate clothing - (unless you love leeches!)
Twelve km south of Berastagi is Kabanjahe, from where you can walk 4 km to the primitive, ancient, traditional village of Lingga where the design of the houses with their horn-shaped roofs has remained unchanged for centuries. Five or six families live in each house, but each has its own family stove. You can stay overnight in a longhouse in a traditional village, (ask for Mr Teger at Café Raymond.) You can go rafting to Bukit lawang - Berastagi is a perfect spot for a variety of day trips to a variety of places, although its main attraction remains its two active volcanoes.
Sibayak, the closest to town, is 2,100 meters high but fairly easy to climb. It takes about 3 hours and the hot springs at the base are a must after you come down. There are several pools of different sizes and temperatures. It is easy to reach by public bus, and if you feel invigorated enough to walk back to town, there is a little used path through the jungle which starts in the village by the Springs. The local villagers will point you in the right direction.
The other volcano, Sinabung, 2,400 meters is higher, steeper, harder work - you need your hands to climb at times - but it is very rewarding, you can see Lake Toba from the summit and there is the pleasantly scenic Lake Lau Kawar at the bottom where you can swim or stay at the campsite. From here you can travel further south buy bus to the Liang Pahar Cave, with its spectacular limestone formations.