Classic play has a unique idea for writing a secret message. Using a paper tube and long strips of paper wrap the paper around the tube and write the message. Unroll the paper and deliver. The recipient will need to re-wrap the paper around the roll to reveal the message.
Good for ages 8 and up.
Get the instructions here:
http://www.classic-play.com/spies-like-us-secret-codes-pt-2/