Take a look at the Hotel Certosa di Maggiano in Siena. I've heard very good things about it.

Hotel Certosa di Maggiano is very good but overpriced, in my opinion. "Il Canto," the restaurant at Certosa di Magiano is worth a trip, however Relais La Suvera, which was the former Summer residence of Pope Julius II, is in many ways a superior hotel as far as the accommodations and the property is concerned. And the rates at Relais La Suerva are more affordable.

I would also look at Locanda dell'Amorosa, which I'm told is absolutely breathtaking.

We recently enjoyed 5 nights at Relais La Suvera - a phenomenal experience!!! We have stayed at many of the finest hotels in the world, and I would put Relais La Suvera at top of our list of our favorite hotels. Beautifully maintained with an ambiance of refined elegance without pretension.

I have discovered this amazing property at the gateway of Chianti called Borgo di Fontebussi. The rooms are magnificent, the place is a jewel, and it is off the beaten path enough for you to enjoy the real Tuscany!
I also stayed twice at Hotel Villa San Paolo. in San Gimignano, is you want to be closer to "mainstream" Tuscany. I loved this property!

You can rent some lovely 2 bedroom apartments very close to Sienna in the countryside. The website is www.vicodarbia.com
I believe the minumum rental time to be is one week. This is on a wonderful farm, olives, vines, corn. We have been going to this location for about 10 years.

If you are considering going a bit further south, I would recommend Castello Banfi il Borgo, in Montalcino. The vineyard to the great Brunello wine, the setting is magnificent, the rooms/suites sprawling and comfortable, great food and, of course, that wine.

Montalcino is a charming town, with very good restaurants (casual) when you are looking for a respite from fine dining.