Day trip w car from Florence

It sounds like you just need to read up a bit more on the Tuscan hill towns before making a decision. Maybe start with the Italy section of the Explore Europe tab on this website. And Rick's Italy guidebook. Cortona would be one of the furthest from Florence- at least 1 hr 15 min each way. Although, if you are going to the trouble and expense of renting a car, I'd want something scenic like the Val d'Orcia, Montepulciano, Montalcino, San Gimignano. Places that aren't easily reached by train, like Lucca and Pisa are.

As for day tripping to a coastal town - it's a LOT of driving for a day trip. On arrival, parking can be a problem. And September will still see the daily inundation by thousands of cruisers on excursions.

Definitely go to see Voltaire if at all possible. Not sure where he is hanging out these days

Those who can make you believe absurdities can make you commit atrocities.

Been to Cortona....full of tour buses with folks looking for her house. I would avoid. That said, think of many of the other small tuscan hill towns, Peinza, Monpulciano, Siena or any other small town. think of going to Chianti and tour tasting and visiting small towns. So much to explore. Take a look at RS tuscan hill town book. Enjoy

I will be renting a car for one day in September (a Saturday).

Make sure you can return that car when you want to.

Agencies will have short hours on Sat- not open for late afternoon returns and some will be closed on Sunday meaning you keep the car until Mon. Parking and driving in/out of Florence then becomes a bigger issue.
Pick up/drop off at the airport might be the solution.

I would drive south of Florence- along the Chianti Road then head to Val D'Orcia for the scenery. Stop in Pienza, Montepulciano, Bagno Vignoni, San Quirico, etc
Those smaller towns are easy to get in/out of with parking outside the walls.

Lucca is not car friendly and can be reached by train.
SG and VOLTERRA are known to be quite touristy/crowded - I would just want to spend the day enjoying the scenery, using the car to full advantage going where trains can't take you.

You’ll have to return the car at the airport (closes at midnight) as the downtown offices will close early.

I would also take advantage of the car to drive through the Chianti road. Once at Castellina In Chianti you can then go to Monteriggioni and visit it for a bit. After Monteriggioni you can head to San Gimignano. If you have time you could also visit Barberino Val D’Elsa, which is pretty and not far.

By the time you are done with that loop, you can head back to the Florence airport to return your car.

Forget Lucca. You don’t need a car to see Lucca. You can go by train.
I would also avoid Volterra. Takes forever to drive there.
You could also opt for the Val D’Orcia as suggested above (Montepulciano, Pienza, Crete Senesi, etc.). It’s a long way out though and potentially you could visit also Cortona on the way back to Florence (take the A1 freeway to go there and return), if you have time. But that would be a very long day..