Here's a classic art lesson that's been taught by art teachers for decades: diy scratch art.
Grade 4 students drew an underwater scene on cardstock. \they outlined the pencil lines with a thick sharpie. Then they coloured in everything using light coloured oil pastels. The colours need to be light!!
Then cover it with a coat of black tempera and let dry. Use a wooden stylus to scratch out designs all over the picture. These look so cool when finished!!