Take footpath to the right of M&S and turn right under motorway. You will then be in the country side. Walk up a slight incline and turn left through a metal gate into the flood defences. 
Plenty of space to run and fetch balls.

Feeling energetic follow gravel path to the end and turn right over a small brick bridge and you will come across a small lake that you can walk round. Why not sit on the large boulders and watch the wildlife - Swans, Ducks, Coots etc

You can also turn left before the lake and walk down to the Oxford Canal tow path.  You'll need to retrace your steps to get back to your car.  No shelter and very open so not good on a wet windy day. Plenty of dog bins.