I love these special devices, You can create your own code and then use the same device to break the code too. First lets make them.
To make the code maker cut a rectangle about 7 cm (3 inches) by 19 cm (7 1/2 inches) from recycled cereal box cardboard.
Fold the rectangle in half so the long ends meet. Snip into the fold at 3 cm (1 inch) from each end. Unfold. Cut into each snip to make a 4 cm (1 1/2 inches) opening centered and running parallel to each short end.
To make the code breaker, cut a 23 cm (9 inches) by 4 cm (1 1/2 inches) piece of construction paper to fit in the code maker's slots. Trim the sides of the code breaker so it will glide easily when you put it into the code maker.
Use pens to mark 1 cm (1/2 inch) spaces to the right and left of the code maker. Write A through M to the left and N through Z to the right in the spaces.
Write the code letters on the code breaker strip so they line up with your code maker alphabet. You can make several codes by sliding the code breaker up so each letter is aligned with a different letter. Write the top two letters that disappear in the new spaces at the bottom. Mark lines on the construction paper strip at the top to show how to line up the letters for the particular message you are sending, then be sure to tell the message's recipient at which line to arrange the code breaker.
These are super cool and the kids will love them. Now how can you have fun with these, you could play a treasure hunt where is clue is a coded message and the kids need to use the code breakers to decipher the clue. You have just made a traditional treasure hunt awesome!