I will be in Tuscany for three nights and would like to stay at an agriturismo to get the full experience of Tuscany. I've looked at a couple different cities/villages in the area and find the Siena region quite nice. I'd like to mention that I have never been here, so recommendations of other places are appreciated.

I'm hoping for an agriturismo that has the following:
- vineyard / wine tasting
- home cooked meals (I need to try true Tuscan cuisine)
- amazing views / beautiful grounds
- pool (bonus)
- horse riding (bonus)
- bike riding (bonus)

I'm looking for a very romantic place for my boyfriend and I. I feel this would give us a very special and romantic stay. We will be renting a car, so we can drive where ever and visit cities nearby. Recommend away.....

>>>home cooked meals (I need to try true Tuscan cuisine)<<<

I don't think you need to eat your meals at a farm to experience Tuscan cuisine.

If you are talking about the views you see on many postcards of Tuscany, most are the Val d'Orica area. You would need to state a per night budget to get suggestions. In season, many agriturismos want a week booking so that might limit your choices.

babycakes,
Try http://www.lacrociona.com/

Marciano is just on outskirts of Siena. Small, friendly. Check tripadvisor for details.
No pool. May be able to arrange horseback. Own small vineyard. Good wine. Have meals several times a week as guests want. Great friendly family style meal. Inexpensive place to stay. Views of Tuscan country side and church steeples of Siena.

On the other higher end, try Villa Vignamaggio between Greve and Panzano. Has everything you asked for. First class place. Mona Lisa supposedly born there. Used in filming of Mich Ado About Nothing.
Website and tripadvisor will give you plenty of info on it. Stayed there a half dozen times. All great.

Terre di Nano
The best region of Tuscany that you see in photo albums.