suze

Another vote for Venice! It is a unique and amazing place. I would think because it is smaller than Florence or Rome, your 2-3 days would go further. Venice is a wonderful place just to walk around, day and night, no planning needed. Take a train into St Lucia, catch a vaporetto to your hotel, it's fairly easy even for a first time euro-traveler (not huge, no traffic, romantic to die for, even when you get lost *which you will* you can't be too far off).

cmt

You'll be so close that you definitely should not miss Florence, which in in Tuscany and which I think is necessary to getting a feel for Tuscany and its history. You should just count of including it. If you want to include another city outside of Tuscany that offers something different and truly memorable, I'd recommend Venice over R0me, in light of your expressed preferences (and my own preference).