The Niš Fortress sits on the right bank of the Nišava river, a short walk from the central square, and it's the most pleasant place in the city to spend a sunny hour. The walls are thick enough to stroll along, the gates are intact, and inside the complex you'll find a mix of Ottoman-era structures, a small archaeology museum, and a large open park where locals jog and walk their dogs.
Inside the fortress walls, look for the Bali Bey mosque (now a concert venue), the remains of a Turkish bath, an old ammunition depot, and several cannons and outdoor sculptures. The archaeological museum is modest but worth a look for the Roman finds from Mediana and Naissus. Entry to the fortress grounds is free; the museum charges a small admission.
The stretch of riverbank outside the fortress walls has been turned into a promenade lined with cafes and restaurants that get busy on summer evenings. From the walls you get a decent view over the Nišava and the older part of the city.