What dates are you looking at for train schedules?

If it's after mid June then the summer schedules may not be posted yet. You can put in an earlier dummy date for the same day of the week, and you'll see that there are hourly trains from Siena to Florence, and they depart at XX:18 all day long.

ready - you are probably looking for a date that is too far out. The schedule isn't usually loaded past June this early.

Pick a day that is the same day of the week you want to travel. For example, you want to travel September 1 this year, a Wednesday. Just enter, February 10 or 17 as your travel day and you'll get the train, times and pricing. They will not change too much when the new schedules are loaded.

Most US folks have aproblem buying tickets on the Trenitalia site. You can purchase tickets as you travel when in Italy.

The trains from Siena to Florence are Regionale trains that do not have reserved seats. There is no point to buying tickets ahead of time. During the week, the ticket counters in Siena open at 5:50 am, so you can buy your tickets just before getting on the train. There are probably also ticket machines.

Just guessing but: Wouldn't it make more sense for the poster to catch a train out of Chiusi or Arezzo rather than Buonconvento?

I actually think Pienza is just too far for a day trip to Firenze.

Agree about the farmacia.

There is a science museum in Firenze.