I've travelled through Bulgaria and Serbia a few times, so here's some ideas:
Septemvri - Dobrinishte - scenic narrow gauge line, periodically threatened with closure. Septemvir is on the Sofia - Plovdiv main line. Never been, but supposed to be well worth a trip.
1970s style loco hauled expresses from Sofia to Varna and Burgas. Don't expect much from the buffet car.
Gorna Orjahovitsa, an important railway centre but close to the fantastically beautiful town of Veliko Tarnovo.
Old 'soviet' electric units.
Second-hand DB stock on many trains now.
Oh, and the 'cigarrette train' from Beograd to Sofia. It's the 0750 departure from Beograd. When I travelled on it in June 2011, the train filled up with shifty characters at Nis (about halfway) who unscrewed the wall and ceiling panelling (including the window frame of my compartment while I was sitting there), filled the cavities with cigarrettes (including the voluptuous folds of the 'scary grandmother' opposite) and then put it all back together. All this under the noses of two policemen and the guard, who passed up and down checking tickets from time to time! At one point we also stopped in the middle of nowhere for an hour after another 'passenger' threw a package towards the station master as we passed non-stop through a country station. Another wad was delivered to the station master at the next station too.
Sadly (for the smugglers) the train was taken apart by customs at the Bulgarian border post, and I jumped onto a waiting Desiro unit at the next platform rather than stick around to see what the end would be!
Coming back there was no such carry on, which was a shame as the journey does go on a bit!