Back in the 8-bit days my brother and I would write little demos, like multi-color sine curve messages and the like.
Most of the time I'd just clean up the desktop after lots of folks messing with it and leave up something useful or interesting like paint one of the games or a notepad so people can type. Or maybe system info... I didn't want to scare folks away from computers, they're cool.
When other people were around, on Windows machines, press the shift key rapidly to make that Sticky Keys beeping sound. They'd always be well confused about the sound!
Most of the time I'd just clean up the desktop after lots of folks messing with it and leave up something useful or interesting like paint one of the games or a notepad so people can type. Or maybe system info... I didn't want to scare folks away from computers, they're cool.