Mount Sibayak is Berastagi's headline trip: an active volcano with a steaming crater, sulphur vents, and wide views back across the Karo highlands. The climb is short by volcano standards but still slippery, steep in places, and easy to misread in poor weather.
Go early, take a guide unless you know the route, and do not underestimate rain or mist. The classic finish is to descend toward Lau Debuk Debuk and soak in the hot springs.