Formatting C
Write a small program that prints "Formatting C:" and starts printing a series of dots at intervals afterwards. Simulate disk access by contiuously creating and deleting an empty text file.

Problems Printing
Write a daemon that sends each individual page of a print job to a different printer on the network. Select the printer at random.

Voice Recognition Technology
Put an intercom inside a machine and then convince some nerd that it is an AI with voice recognition.

Hypnotic Virus
Convince a newbie that there has been a virus going around that presents hypnotic patterns on the screen which can really mess up your mind. Then start up remotely or set to start at a particular time a fractal program of some sort. They'll probably panic big time.

Keyboard Difficulties
Write a TSR that turns the keyboard on and off at short intervals. You'll watch the person try keyboard after keyboard. Can also swap keys using ANSI.SYS or xmodemap depending on the system.

Fun With A Mac Floppy Disk
Rig the spring in a Macintosh floppy drive to fire the disk a goodly distance from the machine upon ejection.