Is it too cheeky to suggest the ice cream parlor in Diagon Alley? Otherwise I'd suggest Leaky Cauldron for more substantial food, or the Boulangerie near the front of the park on the Studios side for coffeeshop-type fare. Would also agree with everyone else that in-park food is more miss than hit, and if you can swing the timing, the sitdown spots aren't as egregiously priced relative to counter-service.