Any recommendations on Agriturismo in Sienna or San Gimignano areas?
Don't know about an agriturismo but tuscanyaccommodation.com shows several. I have stayed at Villa Lecchi in Poggibonsi which is a great base to see Siena, San Gimignano and other areas. The villa is run but a lovely family and is located in a small village just outside the town on Poggibonsi.
In Siena it would be Marciano, often full. Known for elaborate dinners served on the big table in the kitchen.