The one daytrip I really wanted to make from Sofia was to Rila Monastery. It’s one of the, if not the, Bulgarian site most touted when I was doing my pre-trip research. (Rila Monastery even made an appearance in an audiobook I enjoyed, Street Without a Name, by a Bulgarian woman who left Sofia as a teenager shortly after the collapse of the Berlin Wall and returned years later to her much-changed country.)
Lots of tour companies offer day visits to the monastery from Sofia, many of them combining the monastery with a stop at Boyana Church, another UNESCO site. I settled on Traventuria, a company that operates mid-sized motor coaches from near the Aleksander Nevsky Cathedral to Rila Monastery and Boyana Church.