Local kids activities, early August?

Hello, I have a good friend coming to stay the first week of August with her two kiddies, aged 3 and 7.

Still bearing the mental scars from an ill-advised half-term trip to the Natural History Museum, we are loathe to brave central London again. So, we are looking for some fun, local activities, that will result in more happiness all round. What do local parents recommend?

Thanks in advance, Alix x