These day trips are the most popular:
SIENA: this day trip is best done by bus. A car is not needed in Siena.
CHIANTI HILLS ROAD: this day trip is for the wine enthusiast. Private car transportation is recommended since bus service is very scant in this rural part of Tuscany, one of the most famous wine regions in the world. Some people combine a morning spent in Siena with a drive back to Florence in the afternoon. If that is your plan, you need a very early start because it’s a lot to squeeze in a day.
SAN GIMIGNANO: one of the most famous medieval towns in the world. San Gimignano is doable easily by bus from Florence.
MONTERIGGIONI: If you have a car, you can visit both San Gimignano and Monteriggioni in the same trip. But if you use buses only, adding Monteriggioni might be a challenge as buses aren’t too frequent. Monteriggioni however has more frequent bus service from/to Siena therefore you could make a quick visit the day you go to Siena.
VOLTERRA: It’s a pretty town but it’s not easy to visit from Florence without a car, therefore I recommend it only with a car. If you do rent a car, you can also visit Monteriggioni or San Gimignano along the way (it’s only a slight detour). You could visit all three with a car (San Gimignano, Monteriggioni, Volterra) but that would be rushed. Whatever you do, visit Volterra first, then anything else on the way back. Near Volterra the highway is very curvy, so it’s best not to drive it in the evenings. Both San Gimignano and especially Monteriggioni are near the freeway, therefore easier to come back from even in the dark.
LUCCA+PISA: this is best done by train. Go to Lucca first and spend more time there. Pisa is nearby (20 min) but not much to see other than the Piazza dei Miracoli with Cathedral and Leaning bell tower. Leave Pisa for last. There are 60 trains a day from Pisa to Florence and the last one is late (10:30pm).
AREZZO and/or CORTONA: both are doable by train and they are in the same line. If you decide to visit both, start early. Visit Cortona first (the farther) then in the afternoon stop at Arezzo for the rest of the day/evening. All trains will stop at Arezzo coming back from Cortona. Arezzo is the provincial capital city of that part of Tuscany. The Cortona station (called CORTONA-CAMUCIA) is at the bottom of the hill in the town of Camucia. You’ll need a bus to go up to Cortona.