Loads to see and do, try the ashmolean museum which also has a great cafe, Pitts rivers museum and also museum of modern art where there is another great cafe! Try looking up the story museum too, they run lots of events. Botanic gardens too as they have indoor exotic plant houses, like being in the jungle for kids. Worth looking up the oxford playhouse too to see if they have any shows on for kids in the summer, they usually do. You definitely won't be bored!
We're going camping soon near Oxford and had in mind to go to the Oxford Natural History Museum, but I've just seen on the website that they're closed in 2103. Can anyone recommend any other museums or attractions that a 5 year old might enjoy, preferably indoors so we have a back up plan for rainy days.