Long Crendon Playground
If you are fairly pragmatic...oh, okay, enough of the barely begun justification: I meant to go to Long Crendon Court House, but I neglected to look at the National Trust guide book for opening times and found it was closed. However, recreation grounds are never closed and so we went to their park instead and ate our lunch.
My kids really liked it here, There are a few items of equipment I've not seen before: a climbing wall, a vehicle with a slide out of the back and a cool ammonite style flooring in and surrounding the seating areas.
Long Crendon is not in Oxfordshire, it's just across the border in Buckinghampshire. The park is visible from the main road. It is on the right as you are coming from the Clifton Hampden side. There's a lot of parking.
You can find the park on this map.
There are lots of facilities in the village: A coffee shop, newsagents, pubs etc, all obvious from the main road.