Nepenthe is open every day of the year, but closes for dinner on Thanksgiving and Christmas. That means they close at 4:30 pm and I'm not sure if they serve the traditional Thanksgiving fixings that day or not.
Two other possible options might be the restaurants at Ventana Inn and Post Ranch Inn. You don't have to be a guest to dine there, but you will need reservations and appropriate attire. I say "might be" because I'm not sure if either would be open that day. It's too far out to book ahead. I suggest you call them directly to find out.