I've been to Italy a few times and my wife and I went there on our honeymoon 10 years ago. We started in Venice, then Florence, Rome and Positano on the Amalfi Coast. In fact, I bought my leather gloves there (see my other post)!
I love Florence and Rome for the charm and history, respectively. But there is no city in the world quite like Venice. Since you won't be traveling in the summer (when it can be a bit stinky), you really should try to get to Venice. It's wonderfully romantic and really beautiful. Some great restaurants too, but that can be said for all over Italy.
Cameron Crazies Do Not Storm The Court