This Classroom Activity is from the website Lesson Planet–a service for K-12 teachers. It recommends having students learn simple secret codes. The simplest: have them write the sentence backwards. However, you can also try simple letter swaps: Learning How to Send Messages in Code.
Lesson Planet recommends this as a language arts lesson, but there is no doubt that it also helps with math skills. What is algebra, but a letter standing in for an unknown number? They are also important for teaching computer skills–particularly programming skills.
You can even use a lesson like this to talk about history. Did you know that the Cherokee language helped us to win WWII?
You might even talk about poetry as a kind of code. Metaphor, for example, is sometimes used in simpler codes. (Anyone who watches American gangster movies knows the characters probably doesn’t really want a bag full of cannolis.) It is also critical in more complicated codes.
If you’re looking for more complicated codes for older students, you might also check out the following references: