In September we leave Rome for Tuscany on a Sunday. Our plan was to rent a car in Siena to go to our farm apartment. However, the car rental agency is closed on Sunday. Does anyone know if there is a way to pick up a car in Siena on Sunday even if the agency is closed? Or must we go to Florence to pick up a car from an agency near the airport?
In your place, I would contact the management of the farm apartment and see if they can arrange to pick you up at the nearest train station. Another idea would be to leave Rome on Saturday, pick up the car and stay in Siena Saturday night.
Why not pick up at the Rome airport on your way out on Sunday? You'll pay an airport tax for picking up there, but driving from the airport will be easy.