The tour operators were very informative and pretty organized. Make sure you call and verify! I did, as they state on the voucher, and they didn't have my name, so I had to fax the voucher, and everything turned out OK.
They allowed about an hour in San Gimignano - go straight to the tower, climb it, and spend a few minutes, because that may be all you have time for! We then went to a wine tasting nearby on our way back, which was delightful!!
Then we went to Sienna. I recommend the traditional group lunch; it was a fantastic cultural experience!! However, if you don't do that, you'll have time for shopping...which we missed!
The tour guide was awesome in Sienna, but we spent more time in some places we didn't care about, and wanted to skip the end of the tour for some shopping. Do you hear a theme? We just wanted to find a nice bottle of local wine, some olive oil maybe, just check out what things are like...our local guide says that Sienna shops are much cheaper than Florence, too.