Sharks and Fish
This can be improved by making creatures pick one of the four compass directions and always head in that one until their death, although that can result in a shark dying because it's trailing behind another one. A way of improving on that, which just occured to me the other day, is for each creature to pick one of the four compass quadrants to head in (NE, NW, SE, SW) and then pick the ratio of the two directions it moves at random. On each turn, it decides which of the two based on whether it's currently above or below the goal ratio since it was born.
That should result in far less geometric and more realistic looking simulations. I haven't implemented this yet, if someone runs through the exercise please let me know.