Here's my favorite project from A. K. Dewdney's book "The Armchair Universe: An Exploration of Computer Worlds"
The planet Wa-Tor is entirely covered with water. The only two occupants of this watery world are sharks and fish. The sharks eat the fish and the fish exist on a never-ending supply of plankton.
As you will see, this simple ecology is highly dynamic with both sides walking a thin line between continuing life and species extinction.
With this simulation you can modify the biological variables of the shark and fish populations. Can you create a self-sustaining ecology? Or is Wa-Tor destined to be a dead life-less planet?