I teach a unit on positive/negative space each fall as a tool to help my students better focus and draw. I have a number of handouts I use for them to practise turning a positive image into a negative image and I set up a couple of displays for them to draw. I typically use some children's chairs I've painted white and display them on a black background so the students can clearly see the positive and better study the negative shapes.
The unit test for positive/negative space is drawing a butterfly - one half positive and the other reverse as a negative. I encourage my students to "invent" their own butterfly using personal images, symbols, text, etc. My students get to design their butterfly in pencil on plain typing paper or in their sketchbooks. We use a giant light table in my classroom to trace the designs onto nice, drawing paper to finish their drawings in marker. They can only use two colors.
Here are some samples of my student's positive/negative butterflies.