There's no way you could see all three of those towns in a day, even if you had a car.
We based out trip to Tuscany in Greve and visited San Gimignano and Siena by car. The three offer decidedly different experiences.
- Siena is the one most like Florence in feel and size. Like Florence, the historic district is surrounded by a more modern city. It would be the easiest to reach by public transportation. It would take a full day to just skim the surface. Take an early train (it's ~2.5 hours by train from Florence and stay as late as possible.
- San Gimignano is the typical Tuscan hill town. It's gorgeous and can be seen comfortably in a day. It takes 90 minutes to two hours to reach from Florence either by bus or a combination of train and bus. It takes a transfer from one bus to another.
- Greve is the smallest of the three. It is a charming little town with a nice piazza, but no big tourist attraction. We rode the bus from Greve to Florence and back. We were the only tourists on the bus, the rest were locals heading to work or school. It was nice to see a slice of life for the Italians. We stayed at Castillo di Verrazzano, which is a winery that not only produces excellent Chianti, but also olive oil and authentic (and expensive) balsamic vinegar.
A possible Greve itinerary would be this: Take an early bus from Florence to Greve, planning to arrive between around 9:00. The bus stop is a 5 minute walk from the town square. Wander around the main square for an hour or so. Hire a taxi for the 10 minute drive to Castillo di Verrazzano. Take their combined winery tour and food and wine tasting. The tour takes about an hour to an hour and a half and is fascinating. You'll get to see the grounds of the castle, which sits atop a hill overlooking the valley below. The food and wine tasting starts after the winery tour. You'll be served Prosecco to start things off. You'll then be served the first of three courses paired with their Chianti Classico. You'll be served their Chianti Classico Reserva and their Super Tuscan with the other courses. You'll be served scrumptious pecorino cheese, wild boar salami (that they produce), pasta, beef that is cooked on their open flame wood grille, their own olive oil, and their own balsamic vinegar. As they say there, when served their balsamic vinegar, you'll say "Oh my God!" twice. First when you taste it (it is heavenly) and second when they tell you the price! For desert, they serve you biscotti and an excellent vin santo. You won't have to worry about dinner that day.
The food and wine tasting will take the better part of the afternoon. Plan on two or three hours! You could then take the taxi back to Greve and catch the bus back to Florence. There's a small art gallery on the town square and you can pick up mementos of your stay created by the artist owner. Two of his works hang proudly on our wall along with some photos I took on the trip.
