This was our 3rd country visited on a 2.5 month trip. Similar to Romania just prior to this part, we didn’t do much advance planning.
Overall, we enjoyed the people, towns, sights, history, wine, and unexpected mountainous scenery of Bulgaria. Although we started this trip using public transportation, a few days into the trip after a hell-ride on a minibus we opted to get a rental car for the remainder (which also simplified getting to the Rila 7 Lakes area). We did not visit the Bulgarian seaside. Parts of the trip we had intense heat which made it difficult to enjoy some of the areas (we may have liked Veliko Tarnovo more if it was not so hot).
Based on our trip (and copying Steve’s rating system with ^ for ratings) this is how we summarize what to see/do:
^^^ Rila Monastery (1-2 hours + travel time)
Impressive monastery with unique colors in a scenic location.
^^^ Rila 7 Lakes ( ½ day + travel time)
Chairlift ride gets you to the start of a loop hike with beautiful mountain scenery.
^^^ Plovdiv (2 days)
Very impressive ancient city for strolling, eating, and drinking wine.
^^ Veliko Tarnovo (1 day)
Fine historic area sprawled around a valley.
^^ Sofia (1-2 days)
Enjoyable city with impressive church architecture, ruins, and other sights. Maybe worth ^^^ with more time.
^ Ruse (1/2 day)
Enjoyable relaxing town for an overnight/partial day of sights if relocating from Bucharest to Bulgaria. Many day trip possibilities appear worthwhile in the area.