Day trip from Florence to San G and Sienna

As much as San G is a fantastic little village, because you use the bus and it takes more time, I would only do Siena.
There's a lot to do in Siena.
If you deicide to go to San G and buses allow it, I would do it at the end of the day. during the day, there's a lot, and I mean a lot of tourists in San G for the size of the village and it feels like a tourist trap. Suddenly, around 4:30-5pm, tourists buses leave and the place become so much more charming. 2 to 3 hours there will give you a good idea.
Been to both twice. Love them both

You'd do better to take an organized tour out of Florence like:

https://www.viator.com/tours/Florence/Private-Full-day-Tour-to-Siena-and-San-Gimignano/d519-2428P129

I think Siena requires a full day. I would go there first, and if you find you have time left in the day, then go over to San Gim, where in my opinion having been there in March, there is not much to see or do besides shop. I would not spend the morning when you have most of your energy and time in San Gimignano because you may regret it once you get to Siena, which is a much culturally richer and scenic area, with much more to see and better restaurants.

Mary, my recommendation would be to do what you can to be a "contrarian". Meaning that so many people make day trips to both Siena and SanGimignano that those towns, and their various sites, become very, very crowded and touristy, and minimally enjoyable, from about 10 in the morning until 4 in the afternoon. As much as possible, try to be in these towns in the early part of the morning, and then in the late afternoon and into the evening. I have even been known to have a room in one place for several nights (like Florence), and then get an additional inexpensive room in another place for one night (as in Siena), traveling with just a day pack, change of underwear and shirt, and a minimal toilet kit, to, avoid being a "day-tripper".

Three years ago, we did a day trip to San Gim and Siena from Florence. The Siena part included a guided tour of the cathedral, which was very good. Booked the day trip through Viator, although I can't find the exact trip on the website now. I enjoyed it very much, but agree with the others to concentrate on Siena. San Gim is pretty, but also pretty touristy. If you are going to do both, definitely go to Siena first. Have fun!

I would explore Larry's recommendation. I would go to Siena first by either bus or train. Spend a long, wonderful day there, being sure to visit the Duomo and Duomo Museum. (Pre-reserve your tickets - you don't pay until you get there) Then in mid-afternoon, go to SanG and either stay late or spend the night there. We found SanG to be very magical once the crowds left and we could amble, enjoy a meal, etc. Bus back to Florence. For bus information click here. I found this website useful. Between it and Google Maps, you should be able to figure a timeline.

Thanks for the advice. I think we will go to Sienna in the early morning by train and go to San G later in the afternoon. Unless we are too tired. Does anyone know how late the buses run from San G to Florence?