Larry Niven wrote about the end stage version of this in the short story “Flash Crowd” – what do you do when people can teleport into somewhere that’s suddenly interesting, whether it’s a well-reviewed restaurant, tourist spot, or building fire (read the story to find out the solution).
We’ve had pretty good results hitting Rome and Amsterdam in ‘shoulder season’ and crowds were only bad at the Anne Frank house (only because my SO messed up the reserved tickets), and long lines at St. Peter’s. We like walking tours, which let you see a lot of city without fighting to get into any single place – choose your battles. We’re also interested in things that apparently don’t attract huge crowds, such as archaeological sites (Pompeii of course was still pretty crowded), history-of-the-city museums, etc. If you want to see something really cool, go to Vienna, and visit the Musical Instrument museum.