Renting a car in Pisa then exploring Tuscany

I know I need to get an IDL but haven't figured this out yet.

Hi Cathleen -
It's an IDP, not IDL, and here's a way to get it. :O)
https://www.aaa.com/vacation/idpf.html

Editing to add: this free "Driving in Italy" ebook should be helpful for some "things you need to know." Pay particular attention to stuff about ZTLs. (Scroll down to the bottom of the page I've linked to order the book.)
https://italybeyondtheobvious.com

Car rental in Pisa are at the Pisa airport. I think Avis has an office in the city center as well. All major companies (Avis Budget, Europcar, Hertz, Sixt, etc.) are present there.

I, and most people here, use the company below (a broker/consolidator based in Portland, Maine) to rent cars in Italy. Excellent service. They work with all major car companies. Check their pricing. As pick up location select PISA ALL LOCATIONS. To return choose whichever city you want to return, as the default choice is always the same as pick up location. I also select No Deductible option in the insurance coverage menu. Since deductibles are high. You cannot decline insurance in Italy. In your case select also the Automatic transmission option. Auto transmission is not as popular in Italy but many models offer that option. Small Economy size cars do not come with auto transmission, so you will have to settle for at least compact size.
https://www.autoeurope.com/

Learn the European signs based on the International Road sign symbols. The US has adopted only some of the international signs therefore you need to learn their meaning. Most are straightforward and self explanatory symbols, but some are not, like the NO PARKING and NO STOPPING signs.
https://en.m.wikipedia.org/wiki/Road_signs_in_Italy

Learn also about the Zona Traffico Limitato (ZTL), which are restricted zones in the historical cores of cities where only residents with permit and service vehicles are authorized to enter. The road sign is a white circle with a red border, which throughout Europe means NO MOTOR VEHICLES PERMITTED. ZTLs are enforced by camera therefore don’t enter those zones just because you don’t see cops around.
https://en.m.wikipedia.org/wiki/Limited_traffic_zone

Traffic laws in Europe are largely enforced by camera only (like speed). The Waze navigator is heavily used by Italians and it warns you of upcoming speed camera traps. Install it in your phone and use it.
https://www.waze.com/wiki/UnitedKingdom/Speed_Cameras

Regarding ZTLs, we were there in January and saw lit-up signs in red saying the ZTL was closed and ones in green saying it was open. They were situated on regular streets, no red circle around a white disk, but at an intersection where it was easy to just not go that way. I interpreted the green to mean it was alright to go that way at that time - is that correct? So far I haven’t been charged by Avis for a ZTL violation.

ZTL usually operate during the day, Monday to Saturday. When it is lit up green and says ZTL Non Attiva (not active) or Open (in some cities like Florence also in English) it means it is open to all. When it’s lit up red saying ZTL ATTIVA or CLOSED, it means it is accessible only with permit. The white and red circle is also there, although you may not have noticed it all the times. If you get ticketed you will receive a tickets after a few months. First they will send it to the rental company owner of the vehicle. They have to communicate the name of the renter within 60 days. Generally the rental company will charge you an admin fee for sending your name to the authorities. Then within 360 days the City Police will send the notification to your home.