Wander | Classic retro game for the Commodore PET Computer (1978)

This one's a fun little time waster that draws a snake that wanders randomly around the screen. Press SPACE to pause and unpause the worm. While the snake is paused, you can press any other key to change the character that makes up its body. If your timing is really precise, you can abuse this feature to display a message but this is a really hard trick to pull off.

The snake will occasionally box itself in and get stuck but if you're patient, it will eventually manage to uncoil itself and resume wandering.