I found this idea in a book called Make Cards (c 1992) by Kim Solga.
The only supplies needed are: cards (or paper folded into a card shape), a black marker, a black crayon & whatever you choose to draw or create with: crayons/paints/colored markers.
Simply color, paint or draw a design using your medium of choice. Leave an area open in your design for the "secret" message or picture. Here my 5 year old drew a cat. Use a black marker to write your message/picture. Pazely's secret picture is a rat inside the cat's mouth. 
Now, color over your secret picture/design with a black crayon, pressing hard to cover the area completely.
This is how Pazely's crazy-neon-cat-thingie turned out.
The recipient of your card will need to use a coin to rub away the crayon & reveal the secret hidden message/picture.
Here's more of my mystery card gallery...
I used watercolors to create a polka-dotted tea cup.
The message reveals plans for a tea party!
A secret meeting. Shhh!
Here is a pink stove with a secret message baking inside!
"You're sweeter than a cupcake!"
You could even create riddles with hidden answers.
What did the banana do when it saw the ice cream?
