I spent a couple of weeks in Bulgaria last September, traveling on my own. Of the RS tour destinations, I visited Veliko Trnovo, Plovdiv, Nesebar, and Sofia (just one night). It's a fascinating country, and I especially enjoyed talking to some local folks about current conditions there. English is very widely spoken among the younger set.
I rarely buy souvenirs when I travel, but I was unable to resist the inexpensive pottery and costume jewelry in Bulgaria. The craft-shopping street (guide will direct you) in Veliko Trnovo was my favorite spot for jewelry. Some was set in silver; other pieces were just ceramic, etc. I found interesting production pottery in Veliko Trnovo, Plovdiv, and Nesebar. Plovdiv has quite a lot of art at very low prices.
The historic cities have a lot of cobblestone streets. I was tottering around on an ankle I had sprained badly earlier in my trip, and I was very, very glad I was wearing athletic shoes. Those streets are difficult to walk on even when they aren't wet. If it rains...