Florence to Siena by car

San Gimignano and Volterra are not "on the way" rather to the west. San Gimi is not too far but Volterra is. They are worthwhile. Perhaps visit them on another day by rental car.
Take the Chiantigiana, historic route SR 222 from Florence to Siena, passing by or near Chianti villages such as Greve, Panzano, Castellina, Radda, etc. A rental car is needed to tour around Tuscany, for more easily visiting different towns, villages.

Last year my family and friends (6 in total) visited Florence and then drove through Chianti, staying in a B&B 20 km north of Siena. We rented a car at the airport and dropped it off in Siena. Getting out of Florence from the airport was straightforward. We stopped along the way to our B&B, having lunch and tour at Castello di Verrazzano: https://www.verrazzano.com/. We then did wine tasting in the Chianti region, traveled to Bagno Vignoni hot springs, and Siena.

Other than trying to find parking with a huge van on market day in Siena, it was easy driving and navigating. I would do this again next time I visit Tuscany.

Sandy

Driving is not the issue. Parking is !!!! We had a car in and around Florence for driving to various sits. Our son the driver had an app that showed public parking. This was in easy May and parking was a big issue. A lot of the lots are particularily big so you drive from lot to lot looking for that one slot. And sometimes a long hike into town. It worked for us, but it did have a level of stress for all.

We took the bus to Siena where we spent a few days. While we were there we rented a car for a day and drove around Tuscany. We went to San Gimignano, Montalcino and San Antimo. It is easier to rent a car in Siena.

I wanted to rent a car on our way out of Florence. Our hotel in Siena has designated parking for their customers for a small fee so we're all set there.

I have rented a car in Siena and will be driving from there to Assisi, which is our next stop. On the way, I would like to visit Pienza and Montepulciano. However, I was hoping to get in one more day of touring Tuscany, rather than having to deadhead to Siena via train or bus.

My husband is the one who is nervous about renting a car in Florence. I had heard it wasn't an easy place to drive in but it sounds like it might not be too bad as long as we're heading to the countryside and not back into Florence.

The best way to see Siena is by bus. You'll find them to the right of McDonalds across from the train station--and around the block behind McDonalds. They take you into the center city, where the train takes longer and drops you off on the edge of town where you'll have to catch a city bus into the center city.
I advise against going into Siena by automobile as parking is difficult to find, and the streets are very congested. The city is also up on top of a big hill with apartment complexes all over the place.
You could arrange to pickup a rental car in Siena as you're leaving, and San Gimignano and Volterra are good, easy drives from there.

David,

Did you read the OP's additional information just above your reply? She said they had reserved parking at their hotel in Sienna.

We picked up our rental car in Florence one time on Borgo Ognisanti and getting out of town was not difficult at all.

Actually parking in Florence isn't as bad as you might think. Been there multiple times and used parking garages below the street level. Parking in Siena is harder because the city is build between several hils but you can park at the soccer stadium at 4 Piazza S. Domenico. From Florence i would suggest to take the highway no A1 to Figline Valdarno and from there the SP 16 Chiante route). This takes you Greve in Chianti. I you realy want a special souvenir take a very short detour to Via Sant'Andrea, 11, 50063 Figline e Incisa Valdarno FI, Italië. It is a very small shop owned by Sting (singer of the police) called Tenuta Il Palagio Farm Shop
From Greve in Chianti take the SR 222 to Siena.
You can either stay in Siena but why not make it a round trip?

When leaving Siena for Florence i would really take some time to visit Volterra. It is a little over an hour driving but for me absolutely worth the time.
From Volterra take the SR 439 and the SR 429 to Castelfiorentio. From there back to Florence. Roads are curvy but easy to drive.
Hope this helps and if you need more info, contact me.

Regards,

Jerry

Congratulations on planning your upcoming trip.

We stayed in Florence for 4 days. On one of those days, we took an early bus to Siena, picked up the car rental in Sienna, drove around Tuscany for the day, returned the car to Siena, and then took the bus back to Florence. We had a wonderful day tooling around Tuscany and got back to Florence around 7-8pm.

The bus is more convenient than the train, I think, for the Florence to Siena journey.