We went to Siena from Florence this past May, by bus. We did only one museum plus the cathedral, otherwise just walking the town, lunch, ice cream, lots of photos. We took a 9:30 am bus and returned at 5 for dinner in Florence, more than enough time for what we wanted to see, but dinner and some evening atmosphere would be nice to try, too. You can buy bus tickets the day of your trip, I would think, our bus was not full (though summer is probably busier). You could also buy one way bus ticket, stay as long as you want, if you miss the last bus back take the train and buy one way train ticket from machine. We had lunch at Antica Bottega di Siena, a Rick Steves recommendation, just a few steps off of Il Campo. It was good but not outstanding. Enjoy your visit.