Ciao Rachel and thanks a lot for asking my advice on your trip to Tuscany:-)
Castelnuovo Berardenga is really a lovely place to stay, and its location is strategic to see some of the best sights and attractions in Tuscany. Its location in the southern part of Chianti, near Siena, allow you to visit both Chianti, the Crete, and the magic Val d’Orcia countryside. 
Chianti is full of castles and small enchanting hamlets like Radda and Castellina in Chianti, the Crete and the Val d’Orcia offer a very peculiar and incredibly fascinating landscape. You can follow my links to discover each area and also follow this list of day trips you can take starting also from Castelnuovo Berardenga,
Let me know if you need any other tip, I’m happy to help! elena

Thank you so much!  We will definitely use your suggestions. There are so many sights to see and the planning is overwhelming.