Free stuff to do in the Cotswolds?

We will be staying in the Cotswolds for a week or so in September. I have several tourist brochures and it is looking like everytime we do something it is going to cost us heaps. We will have a car and I am wondering if anyone can recommend any things to do that are free (or pretty close to it!). Are there any castles that have free entry? We are interested in doing walks and checking out the small villages (are any museums free entry) and their parks, etc. Please help.