A simple project with nice results. Just put a flower with leaves and stem in an old pasta jar, fill it with water and you're ready to go. Have the students add a baseline through the jar and have them observe that it changes when looking at it through glass it water. Make the initial drawing with a pencil on black construction paper then add white lines on top. Add two other colors (generally a complimentary scheme works better than an analogous one) that lay next to each other to give lines and composition energy.