This is a little idea my dad used to do with me at the kitchen table, and I still do this with kids whenever there's some time to play with drawing. Cut out a picture from a magazine or catalog, and glue it on a sheet of paper. Make a bunch of them! Then draw what's "missing", but don't go for the obvious, draw something with imagination. Get ready for laughter!!
* You can also do other challenging or inspiring things like cut a hole in the paper or glue a solid shape to the paper and try to incorporate those things into a drawing.
Want to see some finished artworks by kids?
Red scraps were glued to paper and then children could choose which paper they wanted to "finish drawing".
Everyone had the same cat glued on their paper. This child turned his upside down and made a balloon with a cat in it. As I recall, this student worked on the computer to draw in the details rather than on actual paper.