You can go a little deeper with teh motions of planets and maybe simulate the orbits of jupiters moons or orbits of asteriods or comets. The short period comets seem to be more interesting in terms of orbits and orientation to the ecliptic.

Or you can find a cool differnt way to shown how Prof. Newtons laws of motion affect bodies in space or how bodies in space are affected by gravity. Though that might be getting to detailed in terms of gravitational forces. But since you said you were interested in balck holes you can simulate how gravity affects light.

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"The big yellow one's the Sun!!!!"