Code Writing and Code Breaking, various levels of each.
These would have penpersonship as a prereq. We could then leave coded messages lying around, mail them, post them, whatever. You have to have CW1 before CB1, etc.
CW1 notes could be read by CB1 and higher. CW3 messages would be only partially readable for CB1's and more readable for CB2, and totally uncoded for a CB3 reader.