Florence and Siena are both nice places to visit. Agree that if you have a car, return it on your way to Florence. The ZTL is challenging, but even more, cars are not necessary at all in the historic center of Florence. The average person could probably walk end to end on the touristy areas - Porta Romana in the south to Piazza Della Liberta in the north - in 40 minutes or so. And most folks won't even venture that far north or south.

Best bet is to return the rental car on arrival, and rent another one if you need it to continue on. However, if you are going to Florence and then to Venice, you are 1000% better off returning the car on the outskirts of Florence and not getting another one the rest of the trip. Venice has no cars on the island, and taking the high speed train from Florence will actually get you further in than a car can, since the train station (Venezia Santa Lucia) is on the island). From there, take a water taxi or the Alilaguna ferry back to VCE airport.

It's more of a joke than anything, but you could actually walk from the historic center to the airport in under 2 hours if you really wanted to. Probably 90 minutes at a fast-ish pace. The sidewalk situation on the route is kind of dicey and is probably the only reason I haven't attempted it. I once sat in construction traffic so bad on the way that I was tempted to get out and walk.