My husband and I will be hiring a car

Ciao Kirsty, while I definitely think that Florence needs more than one day, I wouldn't try and overwhelm yourself with trying to do to much. I would maybe plan to visit one museum (if you're into that) and make sure to stop at the Santa Maria Novella Pharmacy which is a real treat, San Lorenzo central market and the first floor, perhaps a gelato at My Sugar and a coffee at La Menagere. In Pisa, I really like their exhibits at Palazzo Blu, the historical center is well worth a visit, not just the tower. For San Gimignano, I've heard great things about Poggio Alloro (dinner or lunch there). 

Ciao Georgetta, thank you, we will try these places.

You will be going right through Buffalo Mozarella country, so I would recommend a stop at Paestum and one of the farms like Teunta Vannulo. That is not half way, but absolutely worth a visit. You can do a good portion of the "Paestum experience" in a few hours. Further down, if you get off the main highway, Santa Maria Castellabate is a beautiful place to enjoy a lunch along the sea (I recommend one of the places in Le Gatte). Buon Viaggio!

If you want to park your vehicle for your next trip, then hire meet and greet luton