The National History Museum. It is located in a building used as a residence by Todor Zhivkov, Bulgaria's last communist president. It's a nice building with what appears to be a beautiful garden (off limits) around it. Unfortunately, the large spaces and lighting make it ill-suited to be a museum. But that's okay since the museum's collection is -- in the main --uninteresting and spotty and the entry fee is large. (Curiously, tourists give it high ratings. No idea why unless the ratings are mainly from tourist bus people or they have it confused with some museum that we never saw.)
The best thing about visiting the museum was taking the bus out to the edge of town to see it. We asked someone at the bus stop for help, and though he was very serious and did not speak much English to us, he told us how to purchase tickets and, once we were on the bus, how to validtate our tickets with the punch mounted on the wall.