Just travel off-season. I went to Venezia, Firenze and Milano last year in October. Barely saw any more people than I would see anywhere else in the off peak of, say, Dublin. Smelt nice, had no problem with crowds, weather was gorgeous.

I can travel off season because I don't have children, and I can take time off at any point in time during the year. The problem of congestion really is for people with families or who don't can't take time off outside "summer" months.