I went on a one-day excursion recently from Sofia to BELOGRADCHIK and MAGURA CAVE
I used Atlas-S
It was interesting. I was the only foreigner in a group of 60 Bulgarians. It was made clear to me at the beginning that no interpreting would be avialbale and I assured them that my Bulgarian was good enough to handle that.
For the price (32 leva plus entrance fees for Fortress and Cave) it was really good value.
They have a website where you can see other offers including short breaks in Istanbul (see other posts from leedarkwood) and Salonika ("SOLUN" na Bulgarski)