1) Telnet to a box in the other room and play some sounds out of the speaker. Flatulance, or other "interesting" sounds work well. Set this to automagically occur upon the targeted person's login.

2) As suggested by others, use a wrapper to introduce funny messages in place of normal commands. My favs:
"I'm sorry <username>, I'm afraid I can't do that." <username> = dave
"System shutdown in 5 seconds... log off now"

3) alias ls with ls ../ (think about it)

Get your resume ready, as you may be fired over too much stuff like this... ;-)