Siena (bus or train, although bus is quicker) is the top choice.
If you start your day early enough you could even visit San Gimignano on the way back (bus from Siena to SG, then bus from SG to Florence with a change at Poggibonsi train station where you could also take a train or another bus to Florence).
Both towns are the most popular hill towns destinations in Tuscany, and both are easy without a car.
Montepulciano is too far and without a car it is not a good choice by public transport from Florence. Orvieto is along the old rail line Florence-Rome, but also is too far in my opinion. Orvieto is an easier day trip from Rome.