Siena’s worth at least a couple of days. We visited this past October, but only had one night and one day - not enough time. We also arrived by train and had a tough time trying to get into the center. No taxis, and the line of people waiting for a taxi was looooooong and not moving. We finally found the unmarked elevator in the shopping mall across from the train station that led down to the subterranean bus stop, and caught a bus into the old town. No seats available, and our wheeled suitcases rolled and crashed as we stood at the back of the bus and it swung around tight hairpin curves on steep streets. Take a taxi if you can get one, or be prepared to try to hold onto your bag and onto something to stabilize you on the bus at the same time.
Your tour goes from Volterra up to the Cinque Terre, bypassing Lucca (worth a stay of 2 nights or more) and Pisa (very close). Consider going west from Florence to Lucca.
If you’re flying home from Rome, and need to get back there for your flight, you could stay in (or visit) Orvieto on your way. It’s a hill town that’s technically in Umbria, but very close to Tuscany, and an easy train away from Rome.