Depends on how much time you've got. Heidelberg is a lovely old German city, we were in Strasbourg France for two years so could make on a day trip comfortably. (I confess my children liked it because of the American fast food and Reese's Peanut butter cups availability).
Strasbourg rightfully holds the name of the Venice type city of France. Bruges is the Belgian equivalent and by far the most beautiful city there though I'll make Flemmie get peeved as I place Ghent at a close second.
In France Paris is a must...what can you say? Though it's hard to see it, even a glimpse, in less than four days.
Austria, never made it to Vienna, had a campingplatz disaster in Austria, but I know Graz, Salzburg and Innsbruch. Lovely cites.
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I was born under a wandering star.