Advice for a day trip from Florence to Sienna

Bus. Faster than train and puts you on top of hill downtown.
With a rental car, once you factor in the time for the pick up/return paperwork, finding your way out of Florence and into Siena, and finding parking, will take you twice as long.

Info on bus to Siena
https://community.ricksteves.com/travel-forum/italy/bus-from-florence-to-siena-f9c95e90-8a2e-40c1-a530-ef7f17923b27

,,, and get as early a start to the day as you can reasonably manage to beat both the crowds and the heat.

get a car and a driver, ask at your hotel. someone's brother should be available to do this. or find a friendly taxi driver and make a deal.

Siena is my favorite town in Italy. Take the bus as early as you can manage. When they leave you off in the edges of central Siena not on the bus stop the return times. We always made sure we tried to catch the second to last bus at the latest so there was room for error.

The Cathedral is jaw dropping. We have been lucky to be there twice when the floor was uncovered but even when it is covered, they have 2 or 3 of the pictures uncovered to view. The first time I saw it I was awe struck. I had never seen anything like it before although I had seen many norther gothic cathedrals. Magnificent.

Climb the Torre Mangia for a wonderful view over the city: https://janettravels.wordpress.com/2011/09/18/climbing-the-torre-mangia-at-67/

But if that is too daunting, you can also access a high viewpoint through the museum. It lets you out on the unfinished wall of the Cathedral -- building to greatly enlarge the church ended during the plague and didn't resume because of structural errors by architect/builders.

All this was before COVID but I'd assume they would resume it. It is also fun to walk in the Contradi and find the totem fountains of the different Contradi -- such a wonderful place.

The bus is easy and, as noted above, drops you off at the top of the city, right in the middle of where you'll want to be. The bus station in Florence is close to the Santa Maria Novella station. I have taken the train more times than the bus (so that is usually my preference), but you do have to make your way to the top of the city either via escalator or via city bus.

It seems to me that the Porta del Cielo tour is still running https://operaduomo.siena.it/la-porta-del-cielo/ . This allows you to see the Cathedral floor (Some, not all) from above and if IIRC, it includes going to the top of the unfinished wall too.

Reservation is never mandatory, but certainly guarantees you entry and allows you to choose to you the date and time. You can make reservation by sending an email to [email protected] and specify the day and time of visit, type of ticket, and number of people. I think the type of tickets are the "OPA SI PASS and ACROPOLI" tickets.

Please noted that I had this tour back in 2018 therefore it may subject to change.