I recommend that you go for one day to Jivata Voda, not far from Sofia, near village Bosnek.
The village of Bosnek is famous for the spring the Living Water and the cave.
The water from the spring (slightly upward) is captured and flows through the mouth of a sculpture depicting the head of a crocodile or dragon, flowing with a strong stream, then completely drying up. The author and the time of making the tap are unknown.
Beware of ticks. Use repellent before your walk.
GPS: 42.524190, 23.201860