To give kids practice moving objects within a 3D workspace, I came up with the "cubescape" assignment. They draw a cube, turn it into a component, then copy it multiple times and move them around the space in a pleasing manner. Finally they choose a style and send me a PNG of the best viewpoint. Here are a few favorites from the last time I did it:
The assignment is so simple, yet open enough to encourage a good deal of variety. I love seeing how 12-15 year olds can interpret the idea in so many different ways. I haven't yet added elements and principles of design to my curriculum, but this assignment could easily demonstrate unity/harmony, contrast, scale, rhythm, line, and color.