Florence to Siena

This is the best daytrip from Florence and the easiest. The bus takes about an hour and leaves you at the edge of the historical center. There is a bus schedule posted, so you can note times of return; we always particularly note the second to last trip in the evening so we don't run any risk of missing the last bus. Buses run 2 or 3 times an hour all day both ways. The main bus station is next to Santa Maria Novella train station and if you board there you generally get a seat; you can also pick it up at Porta Romana (get tickets at a tabak) but there is more risk of it being too crowded.

If you happen to miss the last bus back (I think at around 8pm), no worries.
Just get down to the station and take the train.The trains run until later in the evening (last train back to Florence departs from Siena at 9:26pm).

As the others have mentioned, travel by Bus is usually the best option as you'll be dropped in the centre of town rather than at the bottom of the hill where the rail station is located. If possible try to use one of the Corse Rapide express buses as they're a bit faster. The bus station in Florence is very close to Firenze SMN rail station, so easy to access. Don't forget to validate your bus ticket prior to boarding.

Are you going there for a day trip or is that the next stop in your route?

We'll be staying 2 nights in Siena, the next stop on our journey. The bus sounds like the best option from Florence. We'll then go by train and a short stop in Pisa before going on to Cinque Terre..