Hint for the next GBOW (don't answer directly):
"You've done it, Neville! You've found the Room of Requirement!"
"It's also known as the Come and Go Room. The Room of Requirement only appears when a person has real need of it, and is always equipped with the seeker's needs."
"So, say you really needed a toilet..."
"Charming, Ronald. But yes, that is the general idea."