Buongiorno,
Well - if you don't mind moving around then, it looks like a pretty sound itinerary.
However, keep in mind you could stay in San Gimignano for the two days - move to Florence dump the car at the rental agency and then use public transport to get around. You don't need the car for your other destinations and you can save money...maybe invest it is an organized tour of CT so you can enjoy it with out the hassle.
However, You can do CT by train (easy and maybe exhausting day trip) and Venice is just as fast (and definitely more convenient) by train.
This arrangement will have you staying in Florence and experiencing it night and day - which is a great way to enjoy the city.
Donna Denise
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