(Well, one of the coolest things…)
This website, brought to my attention by the Bad Astronomer, is just way too much of a time sink. I can sit and play for hours. It is a solar system simulator which lets you play with the initial conditions with up to four different bodies!
As Phil Plait says, start with one of the default solar systems and tinker with it. It’s fun. I especially liked trying to craft odd slingshot trajectories.