One of the cities which your parents will absolutely love is Ljubljana. This incredible city is regularly described the second Prague, because of its amazing architecture. However, it is much less busy and all of those marvelous places and buildings can be seen with no lines . Moreover, even though you would most presumably start such trip from the Ljubljana (as there are plenty of cheap flights to Ljubljana), there are also many other spots which worth to see in Slovenia. For example, in this great country you can see gorgeous Alps and marvellous lake. And all of that is reachable in less than two hours journey from Lubljana. Doesn’t this sound amazing? Your parents will definitely love it!
Another city which can be a great idea for a trip with your parents is Chicago. The generation of your parents very often sees the United States 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.