We'll be staying in Siena for three nights in May, and plan to rent a car on one of the days to see some of the countryside and visit a nearby town or two, with lunch somewhere along the line. Right now, we're considering several different options: (1) Siena-San Gimignano-Greve-Siena, (2) Siena-Volterra-San Gimignano-Siena, or (3) Siena-Montalcino (or Pienza)-Montepulciano-Siena. Any advice on these routes, and is it realistic to think we can do this much in one day? Thanks.
Any of your choices can be done in a day. If you choose no. 2 (or also no. 1) I would also make a quick stop at Monteriggioni. It will take no more than 30 min. of your time just for a couple of pictures and a wine taste inside the city walls (there is a wine tasting room inside the city walls). Monteriggioni is located between Siena and Colle Val D'Elsa and you have to pass by it on the way to either San Gimignano or Volterra.
If I had to choose and that was the only trip to Tuscany, I would choose no. 2 (with Monteriggioni added). The total driving time for your loop no. 2 is probably around 3 hours. So that gives you an idea of how long you can stop in each place over a 12 hr trip.