Day trip suggestions, assuming you have a car available:

- Vratsa with the Ledenika cave. If the weather is good, do not take the highway and use the mountain road through Svoge - plenty of great sights, imposing rock fomations, a monastery, the river, etc.

- Kazanlak and Shipka can be combined with a lot of charming small towns on the way to Kazanlak. Again, do not take the highway and instead pass through Sopot, Kalofer, Karlovo. This may take the whole day though.

- Assenovgrad (very near Plovdiv): you can visit the Bachkovo monastery and the fortress of Assen.

- completely off the beaten path: the Iskar gorge and the caves near the village of Karlukovo. There is a large cave that resembles a tunnel with two eye-shaped holes at the ceiling known as "God's Eyes" - amazing.

- Kyustendil and the Ruen monastery. Kyustendil is a nice small town full of cafes, the monastery is up in the mountain and the road offer great views.