Surprised no-one has said Venice yet. Sure it's overpriced, overcrowded and somewhat out of the way compared to the other places you want to go, but I feel like it's one of those places people consider that you need to see once, if only because of how totally different an experience it is from virtually anywhere else you'll have ever been.
Beautiful, yes. But the food sucks. Worst in Italy. Hit on the next go along with Como and a few other northern areas.