As a last stop of our 2018 meetup we went to Silistra. We come from Srebarna and just followed the Danube.
Els had prearranged a tour (you can't get in otherwise). From the outside you can see a small chapel through the fences, but that's not really what you come for. It's the tomb hidden below the chapel that is interesting. It's interior is richly and colorful decorated and in a good state of preservation.
Still, I did not take any pictures which normally points to a fairly underwhelming experience.
OUV
The site is tiny. It's a single Roman tomb, nothing to really get you excited. I can only see this happening if it were combined in a serial nomination.
From Silistra, you can also cross into Romania to head to the Danube Delta. If you are going by car, you can stop at the l'ensemble rupestre de Basarabi (T). Philipp's review should give you a proper feeling for what to expect there.