Can anyone advise on the best car rental agency to use in Florence? We'll need a car for a week while traveling to Pienza in the the Tuscany region.
Use a consolidator for best prices, such as:
www.kemwel.com
www.autoeurope.com
(They are sister companies but check prices with both)
They work with the major rental companies in Italy (Europcar, Hertz, Avis-Budget)
If you have a car for a week I suppose you won't be staying in a hotel within Florence historical center. The historical center of Florence is closed to non resident traffic.
This link has information on the zones in Florence where it is prohibited to drive with the penalty being a stiff fine. This will hopefully help you plan where to pick up and drop off the rental car.
http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm