go get a fog machine and hook it up to the Arduino and put it to a sensor for example microphone to detect noise or PIR motion sensor. once you get a reading wait 5 min and activate smoke machine. place behind computer. They think computer is blowing up.
This is a great idea, but you don't need to go to the expense of buying a fog machine, you can build your own in a bottle cap. All you do is put a high value resistor in a bottle cap or other small bowl-shaped container, connect one end to a diode to keep the power from flowing back into the Arduino, and connect the diode to a digital pin and the other end of the resistor to ground. You can then put smoke machine fluid, e-cig juice, or food grade Propeline Glycol (Liquid Smoke) in the cap and program your Arduino to deliver voltage to the circuit when you want smoke.
You'll have to experiment with resistor values, and it can take a while for the resistor to get hot enough to produce smoke, so you'll want to work on the timing too. If you want things to happen a little quicker you can use a 9 volt battery and a relay triggered by the Arduino.
WARNING: Do not connect the high value resitor directly to a digital pin without some way to keep the current from backing up on you, unless you want the smoke to come directly from the Arduino.