Got some time to kill with an addictive "game"?
I found these gravity and planetary simulators. The first one's a fun little star system simulator that simulates the orbits of differently-sized planets orbiting a star and interacting with each others' orbits.
The second one, the "powder game', is like an active "sandbox" where different elements do different things:
You're welcome for the thing that will kill your productivity for the day. 