Visual cryptography is fun! I used the idea to make wedding invitations when I got married. We printed out black and white patterns on transparencies along with some other graphics, then when you overlapped the "his" and "hers" transparencies you got the secret message.

It took some fiddling to be effective. I originally wanted a high-resolution image but that made alignment really hard. It ended up being larger pixels at maybe 8 pixels per inch.