There are a few dishes your Mom and Aunt should definitely try while they're here. Like the hot Шкембе чорба (Shkembe Chorba or Belly soup - the one that the EU wanted to ban some time ago). It's delicious, just don't ask about the ingredients.
Then there are Яйца по панагюрски (Yaitza po panagyurski or Eggs Panagyurishte style) - Eggs, cheese, butter, paprika http://www.gotvetesmen.com/images/recipes/thumbs_250/235_26955.jpg
You could also try Каварма - Kavarma (Meat, peppers, onion and an egg, basically ) or Сарми - Sarmas (the grape leaf variety are more traditional this time of the year).
Finally, very common in summer is the Таратор - Tarator, which you probably know already.
Or just yogurt. But as far as I know there are only two brands (which are related) that sell real yogurt with live Lactobacillus bulgaricus - Герма (Germa) and Бор-Чвор (Bor-Chvor). The rest is more or less artificial garbage.
Alas, I can't make any recommendations as to where the best of these meals are served. But I'm pretty sure you'll find decent Shkembe chorba pretty much anywhere.