Planning on a day trip from Florence to Siena. Should we rent a car for the day, take a tour, or catch a bus?
Catch the bus. It's quick, cheap and stress free.
Take the Sienna Rapido express bus from the train station Cheap and quick, 70 min one way. Hourly departures.
Another vote for the bus. A car would probably be the absolute worst thing to do as driving in Florence is a nightmare (and forbidden in the center), and is also forbidden in the center of Siena and parking would probably cost you almost as much as the bus, which would have none of the headache.
If I had it to do all over again, I would bus it. I did the car thing and it's hard to find parking.