Here's a cute little encipher/decipher program with a Tkinter GUI I wrote a while back. It's an implementation of a derivative of the Vigenere algorithm; the algorithm is taken from Laurence Smith's Cryptography: The Science of Secret Writing, Amazon link here. It's a dated book (and the technique itself is very old), so don't expect any serious security from this - just something fun to think about.
For a fairly complete description of how it works, see this post:
NOTE: Apparently, the CGI that controls "contribution" submissions inserts newlines around comments, leaving the code looking very sparse. I think it looks ugly. For a more compact version of the same, see the original post.