A day and night in southern Tuscany

My first trip to Italy is coming up in August, and while my itinerary is coming along well, I need some help with a day of travel in between Florence and Rome. I will have a rental car in Florence and would like to spend a day and night in southern Tuscany, before ditching the car and taking a bus or train down to Rome the next morning. I know I don't have a lot of time, but I really want to drive though the Tuscan country side, stopping occasionally along the way and maybe visiting one or two little towns for possibly a meal or some brief site seeing. I think I would like to end up in either Siena or Montepulciano for the night, but I'm open to suggestions. I've heard the praises of many towns – San Gimignano, Volterra, Siena, Montepulciano, Cortona, etc... and I know I can't visit all of them on this trip, but I would like some feedback on what my best options would be. Is my best bet to experience the picturesque Tuscan countryside to travel through the Chianti region, which would make San Gimignano and Volterra somewhat out of the way? If I went in this direction, is it conceivable to hit up a vineyard or two, Siena and Montepulciano? Or will I only be able to get to one of the towns? What are my other route options? The definite is that I will have a car from Florence and want to drop it off in whatever town I stay in, stay the night and then head to Rome in the morning. I've looked into car rentals from Florence to Siena and it's definitely doable, but I don't know if a drop off is possible in Montepulciano. My trip is planned for the end of August and I would really appreciate any advise in building my itinerary for the day! Thanks,
Heidi