Magic String Puzzle
A length of string, with a square plastic block attached at each end. In the middle, the string is threaded through a piece of metal. Your challenge - if you choose to accept it! - is to separate the string from the metal, and then to re-attach it.
We especially liked this disentanglement puzzle, which - if you get completely stuck - comes with the solution!
It comes in a plastic box, with a smart card outer sleeve.