Here is a simple art idea, focusing on shape and colour choice.
Print off some simple drawings for students to choose from. I find that animal outlines work the best but you could also use pictures of transport, fruit, people, maps of countries (map of Australia turns out well), or other simple outlines.
Discuss colour choice with students. What colour combinations complement each other? Which colours contrast? Discuss warm and cool colours. For my example I have chosen warm colours for inside the picture and cool colours for around it.