Purchase a return train ticket that afternoon/evening without issue - day trip

All of these routes are served by regionale trains and the prices don't change and they don't sell out so just choose the time and duration you want and buy your ticket.
Some of these segments Turin>Genoa and Padova>Venice also have fast trains but generally the time difference isn't enough to make the price worth it - especially with "day of" pricing. The Padova>Venice route is 14 minutes faster and 14 euros more the price/time ratio is pretty clear.

I highly recommend the Trenitalia app which lets you check schedules, buy tickets and track trains on your phone.

For Siena you might look into the bus. It's a little more hassle in Florence to find the bus spot rather than the train station but the bus drops you off closer to the center of town since the train station on the flat.

If you have time in Ravenna and are also interested in stone floor mosaics don't miss the under advertised "stone carpets".
https://www.turismo.ra.it/en/culture-and-history/archaeology/domus-stone-carpets/

Have a great trip,
=Tod

Tod, those under-advertised 'Stone Carpets' from ancient Roman times look AMAZING! Thanks, adding it to our list.