What are some good day trips from Florence? Is Pisa a possibility? Thank you.

I assume you mean by public transportation...

By train: Pisa, Lucca, Montcatini Terme and Alto, Prato, Pistoia, Bologna, Modena, Ferrara, Arezzo, Certaldo.

By bus: Fiesole, San Gimignano, Siena, Greve.

I'm sure there are others I've forgotten.

You can also get to the coast e.g. Viareggio. A lot will depend how far you are happy to travel. It’s also possible to do a Chianti wine tour, but you may need to opt for an excursion for that.

Pisa is very easy.

The question is more about what you'd like to do . Rome is only ninety minutes by train but a day is too short. In the other direction Venice is only 2.5 hours by train and it's also too much for a day.

Yes, Pisa is about an hour away from Florence by train and would make a nice day trip.

Not a really a day trip, but on this Memorial Day weekend a suggestion. A few miles outside Florence is the American Cemetery, a very moving beautiful site. Several Medal of Honor recipients are laid to rest here, as well as Red Tails, Buffalo Soldiers and Japanese Americans with the 442 combat regiment. If you are interested in WWII history this is a worthwhile place to visit and easily reached from the city.

I’d suggest a day trip to Lucca and Pisa by train. Easily done both in a day.

There's some good guided day trips that San Gimignano, Siena, Pisa, wine country.
Some big ones that hit them all, or just 1-2

Siena was terrific.