Make a moving solar system. There has to be a star in the center, at least one planet moving around that star. (better more). The planet has at least one trabant (i.e. moon, satellite etc.) that rotates around the planet.
You should be able to do this using pushMatrix/popMatrix. Rotate, translate, scale.
The above is the minim requirement. Feel free to make this solar system more than a solar system...
Here is a small hint for animating and drawing: http://www.openprocessing.org/sketch/60525