A few notes regarding rental cars in Italy.
It's important to note that for driving in Italy, each driver listed on the rental form must have the compulsory International Driver's Permit, which is used in conjunction with your home D.L. These are valid for one year, and easily obtained at any CAA/AAA office (two Passport-sized photos required, which may be provided by the issuing office). Failure to produce an IDP if requested can result in fines on the spot! Failure to pay the fines when requested may result in the rental car being impounded, and further charges for towing and storage. Have a look at https://it.usembassy.gov/u-s-citizen-services/local-resources-of-u-s-citizens/transportation-driving/ for more information on I.D.P's and driving in Italy.
You may also want to have a look at some of the other posts here concerning the dreaded Zona Traffico Limitato (limited traffic) areas that are becoming increasingly prevalent in many Italian towns & cities especially Florence, which is almost saturated with automated ZTL cameras. EACH PASS through one of the automated Cameras will result in a €100+ ticket, which you won't know about until several months after you return home! This website provides more information - http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm
There's also the issue of parking tickets, high fuel costs, tolls and automated speed cameras including the devious Traffic Tutor system which measures not only instantaneous speed but also average between two points. Violate either or both parameter and expensive tickets will follow.
A GPS and/or a good map are also advisable.