Ciao Maria,
Poggibonsi is just a few miles between San Gimignano and Castellina in Chianti, so if you stay in the countryside to either side of Poggibonsi you'll be very central. I just suggest to not stay too close to Poggibonsi, as the city itself is surrounded by lots of warehouses and factories so it isn't very scenic... but as soon as you head out, you're in beautiful countryside.
So having said that, I'd recommend spending 3-4 days in Florence divide the remaining days into two different apartments... wait, are you coming this September? some places rent by days, other by the week. So first I'd recommend to stay near San Gimignano for one part to explore Pisa, Lucca, Certaldo, SG itself, Monteriggioni and Siena, and then move on to southern Tuscany to the Val d'Orcia to explore that area (Montalcino, Pienza, San Quirico, Bagno Vignoni, Montepulciano, Montichiello).
So near S.G., take a look at the places recommended here:
http://www.discovertuscany.com/san-g...accommodation/
and then for Val d'Orcia here:
http://www.tuscanyaccommodation.com/val-d-orcia/
Are you all moving around by scooter the whole time once you leave Florence? It sounds like a great vacation ;-)
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