If you're going with a travel group I don't know how much time you'll have for going around on your own, but -
In Florence there is a small museum called "The History of Science" which showcases the Medicis' science collections. Their star exhibit is Galileo's finger. It's a cool museum in you're into science.
There is a very small village in Tuscany called Lucca which is one of the most charming places I've ever been. You can walk around the old city which has around 600 churches, or tour the city walls by foot or by bike.
After dinner we were walking around Lucca when we noticed people entering a church, we followed and were treated to an amazing acapella concert with fabulous acoustics inside the old church - so keep your eyes and ears open for something unexpected like that.
Do you like wine? Red wine in Italy is ridiculously cheap and for some reason not as heavy as in other places (it didn't have the added effect of putting me to sleep is what I mean). Enjoy it!
I found Milan to be rather dull, it's a very industrial city with not many places of interest. There was a great Botero sculpture outside the Duomo, but I don't know if it's still there.
Shops close very early in Europe (7PM weekdays, 6PM weekends, Sunday many are closed) so bear that in mind if you plan on doing some shopping.