In Florence, Siena, Lucca and other historical centres is important to know that inner city areas are usually ZTLs (limited traffic zones), meaning that access is limited (in certain hours and/or for certain car owners). The cars rented out by car rental agencies do not have permits to enter the ZTLs areas, so be careful!
Also, when it comes to car parks it is not always easy to find one, and it’s often quite expensive too. Look the parking signs to know the days of the washing of the road when it’s forbidden to park the car.
If you decide to drive your own car or rent a car in Tuscany you will have to face winding streets and (sometimes very steep) ups and downs, so drive with attention and take your time (also for taking stunning photos along the road!).