Bletchley Park, the "home of the codebreakers" — where Alan Turing and co cracked the Nazi Enigma machine — sells "Pocket Enigma Machines" made from a clever cardboard disc inserted into a CD jewel case. It comes with a very good booklet explaining the basics of ciphering and deciphering with Enigma, and with a bunch of fun Enigma-related activities. Proceeds go to the nonprofit that runs the excellent Bletchley Park museum.
The instructions supplied explain how Pocket Enigma works and take the user step-by-step through the process of coding and decoding. Worked examples and carefully annotated figures illustrate how the Key and Message Setting are used, and there is a trouble-shooting table to help with common errors.
For young readers there is also a simplified way of using it called Junior Pocket Enigma making it suitable for all ages who can read and write their own messages.