We would send emails to the office from unlocked machines.

e.g, long detailed emails about my little pony and asking to start a company My Little Pony fanclub.

Or similar for Celine Dion etc.

Change their wallpaper to something embarrasing, or their homepage etc.

Or just send out an email to everyone in the company saying how much you love everyone, and how lucky you feel to work with such lovely people xxx