Take a screenshot of someone's desktop, set it as their wallpaper, then hide all their real icons. Watch the confusion unfold.
Sneak into their phone settings and add autocorrect replacements so common words turn into silly substitutes.
Put a small piece of tape under their mouse covering the sensor. They'll wonder why their cursor won't move!
Print a fake breaking news headline that's outrageous but believable and leave it on their desk.