We're thinking of a day trip from Florence to Siena tomorrow, Monday 13 October (rain predictions interfere with our prior plans). There are many recommendations to take the bus as it's about the same amount of time as the train but let's you off in the center of Siena.
I can't find a current bus schedule that seems useful. Links in forum posts are generally broken or don't have buses at appropriate times. We'd like to leave Florence around 9:00am or a bit later and be back in Florence by 5:00pm.