Ciao SamFel, and welcome to the forum!
I second the idea of renting a villa for the week and then spending time at the villa and on day trips around the region, relaxing as much as you want and taking every day as it comes. It will be a great 40th anniversary celebration!
With 6 adults, I'd suggest renting 2 cars to have both more space as well as the freedom of splitting up the group when there might be different ideas of where to head off to.
A great base would be anywhere in the area between Florence and Siena in the wine region of Chianti -- it is pretty wide so choices are actually quite a lot, it will depend on what appeals to your group the most.
Another great area is just to the south of Siena, making it easier to visit the Maremma area and coast as well.
Most villas are divided into apartments so you can also choose to rent smaller multiple apartments or a larger one for the entire group... or look for the smaller villas where you can have the entire place to yourself.
Some examples below with 1, 2 or 3 bedrooms.... pay attention to the ones that any of those that say sleeps up to 6, because it could be that in reality there are beds for 4 people and the other 2 are in a sofa bed in the living room. These are fine for families with kids but not for adults.
So just to say there are choices - a villa or whole apartment for all or divided up but at the same place.
Podere Cunina
San Giovanni in Poggio
Villa Corsanello - a whole villa
Borgo della Meliana - apartments as well separate villas with 3 bedrooms
Fattoria Viticcio - if you want to stay at a winery
or
Agriturismo Montalbino at a working farm (with restaurant)
Il Cellese (with restaurant)
Just
go here and browse as well - to get an idea of all the beautiful places you can end up at! ;-)
As far as day trips go, I'd suggest:
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San Gimignano
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Volterra
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Siena and Monteriggioni
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Florence (of course!)
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Chianti - hilltop towns such as Castellina, Panzano and Radda - vineyards all around, great views.
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Pisa and
Lucca
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Val d'Orcia - a drive between Montalcino and Montepulciano to see beautiful country with stops along the way
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Arezzo
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Cortona and
Anghiari
There are of course lots of other enchanting villages in between all these, so I recommend having an idea of which ones are there to visit, but definitely staying flexible and heading out when you feel like it!
