@Oneser@lemm.ee My biggest tip is to take a look at 100 nto. It lists 100 interesting touristic objects all over Bulgaria. It leads you to many smaller but great places. Google maps has an slightly outdated overlay that shows these places. At most of them you can get a small Booklet. In it you can collect stamps from each visited place.

Some specific places that come to my mind are:

• Veliko Tarnovo. It's the former capital and has a big castle ruin. In the evening head to the Monument to the Assen Dynasty and enjoy the view of the city.
• Smolyan and the area around it. The city has a great Regional History Museum and the area around the City is good for Hiking. Maybe take a Jeep ride up Eagles eye and visit the Yagodinska cave.
• The Seven Rila Lakes in the Rila mountains. You can take a cable cart up and hike the rest. From there you can also hike to the Rila monastery.
• The Devetashka Cave. It has these impressive arches. Good for pictures :)
• Plovdiv. The historic city center and the student filled bars and cafes.
• Burgas has not only a very nice beach but also the awesome Sea Garden. I would avoid the typical tourist beaches like the Golden Sands near Varna and the Sunny Beach near Nessebar. Instead opt for the central beach in Burgas where most locals go anyway.
• Try to eat at a механа (Mehana) and order Food in a сач (Sach).
• Drink some Boza.

If you have any specific questions like rental cars or acoomodation feel free to ask here or write me a private message.