Cleaning every crevice

“Mr Potter, is there anything else Dobby can do to repay your kindness? Anything at all?”

“Dobby, you’ve really done enough,” Harry wearily rubs his eyes. The house elf’s generosity would be refreshing if it weren't so persistent and excessive. “You don’t need to do anything for me, really.”

“But Dobby must, sir, Dobby wants to help Mr Potter however he can!”

“Okay then, Dobby, why don’t you go clean part of the school.” Harry felt much better about asking for something that others could benefit from, it he only asked for selfish things he would feel terrible inside.

“And what shall Dobby clean, Mr Potter?” Dobby whipped out a rag eagerly, awaiting the specifics of his new task.

“I don’t know, pick something. Just clean every nook and cranny of this wing, or something.”

“With pleasure, Mr Potter!” Dobby snapped his fingers, and - before Harry could reconsider asking for a smaller area - vanished.