Computer enthusiasts generally fall into three categories - Windows, Mac or Linux-based. To generate some short-lived and hopefully good-natured pranks on the Mac users, the following project may be of interest. It's a small Arduino-controlled infra-red transmitter that over a random period of time can send a variety of random controls or random button presses to a desktop Mac. Once hidden on a bookshelf or begind the user, some very frustrating mayhem could ensue.
To get started, see the code and instructions here. To build your own version is easy, and as size is important - consider using the Freetronics LeoStick - one of the smallest Arduino-compatibles on the market with onboard USB, RGB LED and piezo: