I told him to draw a "secret" picture using the white crayon.
He smiled the entire time and kept saying things like "I bet you can't guess what it is". :)
You can draw with colored crayons too. For my college project, I had to draw an entire picture with crayons (leaving some spaces white) and then paint. It resists either way. I just think using a white crayon is more fun for kids.
After you have drawn your picture, paint over it using water colors. Little Man also loved this (and did in the past as well) because the water colors are actually mine. He would always ask if we could use my "special" paint. Take that as a lesson too--if you only allow things every once in a while, they become much more special. How much are water color paints? 99 cents?? :)
Here is his picture. It's sort of hard to see the drawing because he didn't color to hard.
It's a star wars picture...droids to be exact.
I also drew a picture and made sure to color really hard/go over each line.
Then I let Little Man paint over it.
He enjoyed finding out what the "secret" picture was that I did.
He made sure after we were done that I wrote on the bottom "Drawn by Bethany, Painted by Little Man" :)
Just for some more ideas--when he was 3 and we did this, I knew he wouldn't draw a picture. Instead, I drew the pictures for him. He was really into construction type trucks at the time. I found one of those simple books he had and then just traced the backhoe, excavator, dump truck, etc. He was also learning how to spell and write his name, so I did one with his name written out too. You can always incorporate learning into it.