Encoding
Look at how everything can be encoded, and how one needs to know how to decode stuff.
Examples
English <-> other language is encoding
Music notation - encoding sounds
Numbers - encoding quantities
Try to encode different things
* Words - make them impossible to read without the key ( "X + )(" scheme)
* Pictures - split into small parts
* Letters - use numbers
Pass information around from one party to another, have the other party decode. Have the pirates intercept the message and try to decode it as well.