Secret Messages and Codes

I'm looking for suggestions of phrases, poems, rhymes, etc. that older elementary kids might recognize, but not know well enough to decode from the first couple of word.

I've been working, most of the day, preparing for a program on codes and secret messages. Since I knew very little about this topic I had to do a bit of research and I've become completely engrossed. As a child I tried making up symbol alphabets, and once, in middle school, another spoken language. But I didn't know about the history of codes. I still don't know a lot, but what I'm learning is pretty cool.

It's interesting to think that language in general is a code. I've been encoding various phrases and sayings using a variety of ciphers, but it's amazing how often the original can be seen as an encoded message. Every idiom requires another layer of knowledge. And what is chatspeak or leet, if not a code? I firmly believe that every group of people has their own code they use to communicate with. Librarians are a perfect example of this, what with "stacks" and call numbers and marc fields. However, groups of friends develop nicknames for people, places and activities that sound like code to the rest of the world. Hey, do any of you know what shiking means?