Off the top of my head:
1: These towns are beautiful and would be a wonderful visit. BUT they are nearly 2 hours away from Florence by car. So it will be 4+ hours of driving and probably more.
2: I found San Gimignano to be lovely but way too touristy to be really enjoyable. Siena deserves a whole day by itself and doesn't require a driver but it does provide convenience. If you do this one I would do a couple of hours in SG as early as possible and then spend the rest of the day in Siena.
3: See above but this will give you the most variety with the least amount of driving.
If you have time to do Siena by yourself via bus or train I would take 1 or 3 and do that. If not I'd probably do 2 as I described above.
Is there a guide or just transportation? Some of the towns really benefit from some background and context that hiring a driver will not provide. I would do some research and delve into whichever towns you decide to visit to get the most from your time.
My $.02, I'm sure others will have other opinions,
=Tod