Yikes! The mind reels ... so many places! You don't give an exact timeframe, but any time in summer it'll just be you and 14 million of your closest friends in Florence and Venice. Wall to wall tourists, for real!
First suggestion - get on the Florence Museum website http://www.florence-museum.com/
and get tickets for the Uffizi and the Accademica - you're not supposed to take pictures there, but you can't afford to pass them by, and the lines are ridiculous without reservations.
Get out at night - both places are beautiful after the sun goes down, and safer than most US cities.
If you've got a car, consider the countryside - Ravenna is worth a visit, http://tinius-photo.com/Roger2/Ravenna/
and it's between Florence and Venice. There's Verona, the Dolomites http://tinius-photo.com/Roger2/Dolomites/
, and hundreds of other beautiful places away from the Autostrada.
Germany ... sigh. Heidelberg, Speyer Cathedral, Strasbourg, Colmar ... I'm jealous, and I live in Hawaii!