Back to the computer for some more advanced antics. This one may be more suited for a close friend or significant other, as you’ll have to install something, and you could probably get fired for doing it at work.
Set up a VNC (virtual network computing) server on their system. You can find free ones like TightVNC for Windows or OSXvnc for Macs. Once you get through the configuration, you can click, type, and do anything on their system from your own computer.
Do some subtle things like occasional keypresses or program launches and see how perplexed they become. We don’t recommend keeping this up for long, though, or you may suffer serious consequences with their anger (and you may also witness some disturbing pornographic habits as an unintended side effect).