How about a stay in Fontodi’s vineyards in Panzano, near Greve, about half-way between Florence and Siena?
We’ve stayed as the guests of a family who rented both villas and some of the apartments each summer. It is fantastic. And you can always go to Dario’s in Panzano or to Greve to eat or buy provisions.
Are transport logistics a factor? i.e. are you planning on a minibus to from the airport, but thereafter wanting to be in walking distance of restaurants etc. and using public transport when needed…or are you hiring a multi seater minibus / separate cars for the whole trip?
There will be a huge range available. Two sites I use are:
agriturismo.it aggregates farmstay locations across Italy and there will be a good number that will cater for larger numbers
Special places to stay / Alisdair Sawday, which focuses on the quirky and characterful as something of an antidote to hotel * ratings that drive a more identikit experience.
In addition, Tuscany is a big region. What locations are drawing you to the region? e.g. do you want to have the option of an easy day trip into Firenze, get some winetasting in, enjoy driving through the picturesque countryside, visit some hilltop villages or something else?