This is so easy to do kids love it and it can be turned into all sorts of cards etc.
Have a flat piece of tin foil and with a brush paint the surface. Then use Q-tips or cotton wool buds to wipe off small amounts of paint so the tin foil shows through. Once the tip is no longer picking up colour use the other end.
Once they were dry we turned ours into Easter eggs but you could do Christmas cards or just patterns what ever you fancy.
I cut out the egg shape from the card and then put it on top of the tin foil.
Going back through old photos tonight I found some more pictures we had made in the same way.