Tina,
If you haven't driven in Italy, a few important points to keep in mind....
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. (so you must carry both). 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).
You may never be asked for an IDP, but failure to produce one if requested can result in fines on the spot! Failure to pay the fines on the spot can result in the rental car being impounded (which will result in further financial penalties). Have a look at https://www.ricksteves.com/travel-tips/transportation/rental-car-requirements for more information on I.D.P's and driving in Italy.
You may also want to have a look at some of the posts on the forum concerning the dreaded Zona Traffico Limitato (limited traffic) areas that are becoming increasingly prevalent in many Italian towns & cities (especially Florence!). Some of these are enforced by automated cameras and some by local police. Each pass through one of the automated Cameras will result in a €100+ ticket and visitors often don’t know of these violations until several months after they’ve returned home. In addition to the actual fines, renters will also be charged by the rental agencies for providing information to the authorities. You may find this website helpful - http://www.slowtrav.com/italy/driving/traffic_cameras_speeding.htm
There’s also the possibility of fines for driving in bus lanes, parking tickets, tolls and speed cameras including the devious Traffic Tutor system which not only monitors instantaneous speeds but also average between two points. Violate either or both parameter and expensive tickets will follow!
A GPS along with a good Map is also prudent. Be sure to give some thought to the question of CDW and theft insurance. In Italy, buying CDW from the rental firm is usually compulsory, so coverage provided by credit cards will not apply. Check the Car Rental guide at http://www.gemut.com/ for more information. Their free "Car Rental" PDF Guide can be downloaded. You may also want to check the “Travel & Transportation” pages at https://travel.state.gov/content/passports/en/country/italy.html for other tips (look under "Travel & Transportation"). There’s also this website - https://it.usembassy.gov/u-s-citizen-services/local-resources-of-u-s-citizens/transportation-driving/?_ga=1.21563186.86862954.1474991758
Happy motoring!