One of the places which your parents will certainly like is Ljubljana. This amazing city is repeatedly called the second Prague, because of its outstanding architecture. Nonetheless, it is much less busy and all of those incredible places and buildings might be seen with no lines (useful source). Additionally, even though you will most possibly become such trip from the Ljubljana (as there are a lot of cheap flights to Ljubljana), there are also many other places which worth to see in Slovenia. E.g., in this small country you can see beautiful Alps and marvellous lake. And all of it is reachable in less than 2 hours journey from Lubljana. Doesn’t it sound incredible? Your parents will absolutely love it!
Another city which may be a great idea for a trip with beloved parents is Chicago. The generation of your parents really frequently perceives America as a rich and unavailable country. Actually, it makes sense if we realize that most of people in their generation were going to Chicago not as tourists, but in order to find an employment.