If you are doing a tour of Bulgaria and are in the Ruse area then the Rock Churches of Ivanovo are worth trying to fit in. Their cliffside location is quite impressive (see photo – the main caves are behind the glass window in centre) and the murals are interesting for their realism (no photo as Els only allows 1 photo per reviewer but the UNESCO site photo gives an impression of how they cover the walls and roof!) If you are visiting monasteries like Rila as well you may get a bit overwhelmed with murals unless you are an “expert” but these in some way act as a nice counterpoint to those in the full sized monastery!
We had a rentacar and it was an easy drive from Ruse. But be warned, they are not always open. The external view as per my photo is hardly worth a major journey by public transport (the caves are also a few kms walk up the gorge from the village). The day we were there luckily a coach party of Bulgarians had arrived and the couple who look after the churches were present. If you do arrive and the churches are closed the guardians lived in one of the houses across the stream (The River Rusenski Lom) from the cliff so you might persuade them to open