In the Three Broomsticks, one of the things that you hear from up ahead should be...
Girl: Hey, I have a secret.
Girl 2: Ooh, what? Mum's got some Bertie's?
Girl: No, *whispers*
Mom: JOANNA! KATHLEEN! GET YOUR LITTLE BUMS ROWLING OVER HERE!
Or, on the mirrors in the Room of Requirements, there should be a carving that says, "JKR WUZ HERE,"
Would make my day, really.