Something for the not to tech savvy today.  A fun and annoying thing to do to those around you is just unplug some cables.  The mouse and keyboard are always good.  The monitor can be an obvious one but that makes it no less fun. 

One that is really hard to track down only works if your associate has a powered USB hub of some kind, often a monitor.  You can unplug the USB cable that feeds it but not the power.  Any USB powered device will detect a USB connection but the computer wont.  This can be difficult to track down since the device sees USB power which looks like a connection.