When I was in grammar school, I used to have fun with cryptography – making alternate alphabets with runic characters and hiding codes in scribbles – mostly to pass notes in class with friends. Of course, I had to educate my friends so they could decode messages, but it was always pretty easy. (One kid got caught with a cheat-sheet and the coded-note passing ended promptly.)
Here’s an example of a super-easy message. It’s impossible to make it easier without totally giving it away.
P.S. I guess some people don’t know what runic characters are. Runic characters were the symbols for whole words or individual letters in early written languages – most commonly from 1000-2000 years ago. Here’s a sample runic alphabet: