We could do a playfair cipher. It was my favorite cipher in my computer cryptography class.
http://en.wikipedia.org/wiki/Playfair_cipher
We could leave a clue in our post as to what the key word is and what is used to decipher it.
Example clue:
In a recent interview with the one and only Mario, he was proclaimed to be the savior of the Mushroom Kingdom. Mario humbly replied, "I am the key to the peace in Mushroom Kingdom. But, Bowser keeps coming back and each time with dirtier tricks. Why can't he just play fair and leave Peach alone?" The interviewer continued to tell the back story of Mario, "******ENCODED STORY******"
Mario looked at the interviewer with a perplexed look on his face not knowing what the hell was just said.
Can you help Mario find out what was said?
Obviously, we wouldn't have the bold words in there as that would be too easy.