Assuming you don't expect a Thanksgiving sort of meal since it really isn't that common (there are a few restaurants that will offer a Thanksgiving meal for expats in Florence), then you have a lot of choices for great meals out on that day!
How about exploring the Chianti wine area and ending up for dinner at the
Osteria di Passignano?
If that doesn't fit your tastes, take a look at some of the other places we recommend in the
Chianti area for meals.
I would say just make sure to get in touch ahead of time with them and make sure to book - for 10 people is not a small group but you also want to make sure they are open. While Chianti is not closed for the season, it is the off season and many people choose to take their vacations during late fall and winter considering they don't close in the summer at all.
If you've never been to
Siena, I'd also consider it a special all day sort of day trip from Florence you could enjoy. I especially love the
Duomo and the
Palazzo Pubblico, you can also add in the climb up the tower if you want the extra exercise ;-). The frescoes inside the Palazzo Pubblico on Good and Bad Government make the visit very worthwhile. As far as eating there, one of our favorites in the
trattoria da Nonna Gina or if not that,
Boccon di Prete.
If you don't want to spend the entire day in Siena, you could consider a stop before there, at Monteriggioni, for example, or San Gimignano for the morning and Siena the rest of the day. Although I would really suggest a whole day in Siena, there is a lot to be said for enjoying it without being in a hurry.