Again, we didn't stay in Gjirokaster long enough to explore everything but I heard from friends that Ali Pasha Bridge is a must-visit if you have the time. It was part of a large aqueduct during the 19th century and has been used by farmers and shepherds for a long time.
The bridge is a 30min hike from Gjirokastër and makes for a nice off-the-beaten-path destination.