Correction: we might have taken two buses, not a bus and a train. I don't remember for sure.

http://www.sitabus.it/en/

Maybe it depends on one's idea of crowds, but to me SanG was crowded mid-day in October.

A great feature of the Streetview mode in Google Maps is that it shows what month the view was captured. And if you look at the shadows you can sometimes tell whether the shot was taken in the morning/afternoon, mid-day, or evening. Most of the SanG views were taken in June/July. Certaldo Alto is so small that isn't much available in Streetview, but there some nice views of Monteriggioni.

Keep in mind that in many of the small towns, everything will shut down in the afternoons. Try to visit them early in the day or after 4:00 (if return transport is still an option).

Thank you all. I've done some more research and i think i'll take the Certaldo + Siena combo.
San G, Lucca and the others will be visited on the future!

I plan to leave Firenze at around 9/10am, wander around Certaldo, take some pics, sit on a cafe and grab a snack.

Then i can take the train to Siena (is it better take the direct bus from Poggibonsi to Siena, since once in Siena train station i will have to take another bus anyways?) and spend the whole afternoon at the city.

I'll book the Gate of Heaven visit and then watch the sunset on the famous plaza.

any other thing i should consider? any more advices?

I would take the bus as Piazza Gramsci is far more convenient for the Campo and city centre (a 5-10 minute walk at most).

If you do get the train, you can get to the top of the hill by a series of escalators in the shopping centre directly opposite the station entrance, but that still leaves you about a mile and a half from the Campo.