If you visit Eastern Bulgaria, then "The Wonderful Rocks" would be a must. It's a very rare rock-phenomenon located on the shore of Tsonevo dam (about 80km from Varna).
The dam is quite big and you can actually spend a whole day walking around and seeing different amazing landscapes.
I would suggest visiting "The Wonderful Rocks" during the spring or in the summer as otherwise it can be quite chilly with cold winds, and you won't probably have such a great experience.
In the village of Asparuhovo (about 3 km from the rocks) you can find quite good hotels and guest houses, located on the dam's shore.