For a last minute art project yesterday I had my class make Scratch art. Have you tried this? Its really easy.
1. First, completely cover a piece of paper in coloured pencil (markers would work as well).
2. Second, cover the colourful design in BLACK crayon. Paint works even better, but you need to give the art a day to dry, and my kids don't have that kind of patience this late in the year.
3. Third, use a sharp object to "Scratch" away the black, revealing the colours below. I used my finger nails, some kids used toothpicks, and pencil ends. I'm sure there are other options as well.