We're planning on Rome, Florence, and Venice
Doing a tour? or on your own?
I did the Globus tour of Rome/Florence/Venice. Amazing trip.
If you aren't doing the tour and you want to go to a museum--especially Uffizi in Florence--make sure you have reservations, or you will spend hours in line. Granted, "The Birth of Venus" or Giotto's "Madonna Enthroned" are worth the wait, but if you have limited time, reservations are a must.
Don't let anyone talk you out of Venice. I loved Venice. Make sure that you go two or three blocks off the "main drag" and then start walking. Get lost. It's an island, you can't get that
lost. Absolute great way to experience Venice.
Start walking a couple months before your trip. You will be happy you did.