Stowe Landscape Gardens in Buckingham owned by the National Trust. http://www.nationaltrust.org.uk/stowe/ which has undergone a big refurbishment recently. It has lakes, temples, woodland to name but a few highlights. It is a really enjoyable walk and a wonderful place to have a picnic if the weather is nice and the children have their own trail they can follow which can include playing pooh sticks on one of the bridges. I believe they even run some photography courses for National Trust members.
I took some photographs there recently. Here are some examples