Day trip to Florence

Personally I would drive, but these are the options:

1 - Drive to Piazzale Di Porta Romana, pictured here. The ZTL camera is about 50 meters passed that city gate. Enter inside the city walls using the smaller gate to the left of the large central arch. The Parcheggio Oltrarno (city parking lot) is along the city walls, on the inside, on the left of the photo. Basically when you enter the walls you immediately veer to the left and enter the parking lot (2€ per hour). If you enter through the door to the right of the main large arch, you can still do it, you just have to u turn on Piazza della Calza, just passed the door and then turn right into the parking lot. The main large arch is one way street only to exit the city walls. As I said the camera is 50 meters passed the main door after Piazza della Calza (and you will definitely see it). To reach Piazzale Di Porta Romana, just use Google maps directions. They work correctly. From that parking lot you can walk to the city center.

2 - Drive to Poggibonsi then take either the bus or the train to Florence. If you take the train the next station is Certaldo, which may be closer to where you are staying. Either one works. Be aware that trains or buses don't run all night, so you need to make sure you catch the last one back. Use the Trenitalia website for timetables.

Actually we are staying in Poggibonsi so the train or bus sound perfect.......which of those two would provide the preferable drop off in Florence? I assume the train is faster and will check schedules as you suggest.

The bus station is across the train station (Santa Maria Novella station), so same place.

The train is not necessarily faster. The bus takes the shorter route via the freeway, while the railway goes through the flatter Elsa valley via Empoli, which is a bit longer. However the bus has to deal with traffic too, especially at commute time.

I second riding the train or bus. We took the bus from Greve to Florence. The train and bus station are a 20 minute walk from the Duomo and the heart of Florence.

Be sure to get an early start, there's so much to do! We caught the 6:50 bus.

Train schedules are here. The train takes 64 minutes in the majority of cases (or more).
Enter From: Poggibonsi-San Gimignano
To: Firenze S. M. Novella:
http://www.trenitalia.com/
Last direct train back departs at 20:10. But there are also trains departing later (as late as 23.07) with change in Empoli.

The bus takes 50 to 55 minutes. The bus is no. 131.
Enter Departure: Poggibonsi
Arrival: Firenze
http://www.busfox.com/timetable/
Last bus back departs at 21:15.

This is the PDF version, however these change seasonally, so depending on when you go there might be a new one. So the site above will be more reliable. But to give you an idea, these are the bus runs.
http://www.tiemmespa.it/var/ezdemo_site/storage/original/application/91dc6a056f290d917a5e4a864b34e96b.pdf
http://www.tiemmespa.it/var/ezdemo_site/storage/original/application/81a9b01138449e935a383001e63fade1.pdf