To occupy my nephew for a few hours I asked him to solve a Rubik's cube that had been on the mantelpiece unsolved since 1992, my own childhood. He solved it in a minute, he's still bored, and I am now also resentful.
@fesshole Depending on age/interest, ask him/her about dinosaurs, planets or quantum theory.
@IncHulk @fesshole Or ask for tips on how to solve the cube.