Will it be too much to visit both San Gimignano and Volterra as a day trip from Florence? Is there a direct bus from Florence to Volterra?
The problem is that most towns don't have good bus service between each other. Getting to/from the towns via Siena or Florence is fine. But check the SENA bus schedule to verify.
The SITA bus out of Florence has very good bus service to San Gimignano and Siena (www.sitabus.it). However, they don't go to Volterra. From San Gimignano, the bus travel get more complicated. I'm not sure of the bus line but I think you will have to back track to Poggibonsi and then possibly change buses again in Colle Val d'Elsa.
We did it, but we had a car. Maybe you can find a tour in Florence that will take you to both places.
We did it too and also had a car. I think it might be tough without a car.
We also did it with a car. The problem with a bus is you can get there but you are at the mercy of bus schedules and lose too much time. If at all possible, consider a rental car for the day (and take a GPS).
You will be very happy to visit just San Gimi. There is enuf to see and cafes to enjoy for your daytrip out of Florence. Do not rush this beautiful hilltown. But if you decide to rent a car by all means see both as you will have alot more time. Driving in Tuscany is a cinch but driving in Florence a bad idea. Done it 2X, horrible traffic, drivers and police traps in Florence.