Siena
The best day trip from Florence, and the one to take if you only take one. Siena is a complete medieval city inside its walls — a cathedral, a shell-shaped piazza that the city is built around, and streets that go on being interesting after you have seen the famous things. That matters more than it sounds: most day trips from Florence are one monument with a town attached, and this is the reverse.
Go by bus, not by train. This is the practical heart of the Siena decision and it inverts the obvious. There is a direct regional train, and Siena’s station is around two kilometres north of the historic centre and well below it, so you finish the journey with a bus, a chain of escalators through a shopping centre, or a steep walk. The Autolinee Toscane rapid coach leaves from Florence’s bus station beside Santa Maria Novella and sets down by the walls, minutes from the centre. Same city, materially different arrival.
Do not pair it with anything. Siena is routinely sold as half of a day that also takes in San Gimignano, or Chianti, or both. Those days exist and they work, but they give you an hour or two in Siena, which is enough to see the piazza and not enough to have been there. If Siena is the reason you are going, give it the day.
