The Boyana Church is a medieval Bulgarian Orthodox church situated on the outskirts of Sofia, in the Boyana quarter. It is protected by UNESCO for its frescoes from 1259. They represent one of the most complete and well-preserved monuments of Eastern European mediaeval art.
Other free hike tours - details check here:
Lozen mountain free "Pancharevski Gorge" - 14km/8.5 miles, 6 hours, July-September: every first Sunday.
Vitosha mountain free "Vitosha - City to Summit" - 10km/8miles, moderate, 7hours, June-September - every last Sunday.