Online Gravity Simulator is a sandbox that let you experiment with gravity, planets, star, and asteroids to see how they interact with each other. Give them an initial push and see where it goes. Amazing simulation game that let you visual gravitational attraction in real time.
Why stop there? Go ahead and simulate what we might observe if gravity deviates from a 1/r^2 law. Change the law just a little, and try to recreate the strange and famous behavior of Mercury.
Throughout it all there will be beautiful and captivating pictures. So go make some worlds, and make some art.
As Always, Good Luck and Have Fun.