Parking in Florence

this may help.

http://www.visitflorence.com/florence-maps/parking-lots-in-florence.html

Where not to park: Not in the Flornce historic center close to the ZTL's.

Others may know whether it's possible to take the bus from SG to Florence.
We took the bus from Florence to SG a few years ago, less hassle than driving.
I'm guessing you can do the same thing in reverse, certainly your hotel will be able to advise you.

Do not do not do not drive into Florence if you don't know what you're doing. You'll more than likely get zapped in the ZTL zone and end up with tickets (and admin fee) charged to your rental car credit card. Yes, the bus travels from SG to Florence.

this post was posted twice in the same Forum. Roberto had some interesting insight in the duplicate thread.

Thank you all for your replies. I did see Roberto's response too. Sorry for the double post.
I would rather not drive, however, we only have the day in Florence. The train seems the best option. Have done it previously. The only drawback is the last train from Certaldo from Florence is around 8 pm. Or is there a later train or bus to San Gim?

The last bus to Poggibonsi departs Florence at 21:15 hours.
The last train to Certaldo (or Poggibonsi) departs Florence at 20:10 hours if you want no train changes, but if you are willing to change in Empoli, you can depart from Florence later (as late as 23:07) by taking a train directed to Pisa, then at Empoli you get off and change to a train going to Siena, which stops at both Certaldo and Poggibonsi. Actually the 23:07 option require a change to the bus service, since that late at night, the train company replaces the train with the bus, due to low passenger volume.

In any case, since you intend to stay till late in Florence, maybe to enjoy dinner in the city, I think driving to Porta Romana would provide the best option. Piece of mind and no worries about having to be at the station on time to catch the last train. When you are done, however late it is, just walk to Porta Romana and drive away. From Ponte Vecchio it's a straight 10-15 min walk to Porta Romana.

The name of the parking lot at Porta Romana is: PARCHEGGIO OLTRARNO (PIazza della Calza). Enter inside the city gate and immediately turn left (if you go straight past the little Piazza della Calza, there is a ZTL camera). The parking lot is immediately on the left, on the inside of the ancient city walls.

This is the Porta Romana (city gate) seen from the Piazza outside. Enter inside the city walls and the parking is on the left side along the walls.
http://www.residenzailcarmine.com/gallery/pg_pr.jpg

this is the entrance to the parking lot:
http://www.firenzeparcheggi.it/assets/images/oltrarno/calza1.jpg

Nothing fancy. Just a bunch of parking stalls along the inside of the ancient city walls.
http://www.firenzeparcheggi.it/assets/images/oltrarno/calza2.jpg