My daughter received a spy gadget at a fast-food restaurant some time ago. When she lost interest, I opened it up and connected it to my Arduino.
When you switch on the spy gadget it will give some kind of alarm when someone passes the ir-beam. When this happens, the firmware in the Arduino sends a string over serial to a program I have on my laptop. As soon as this program receives the string, it plays a little sleighbells tune.
I had the gadget positioned in the window pointing in our street during Christmas. When people passed, they were very surprised to hear the sleighbells play.
Here is a low quality video, made with my mobile phone in bad light conditions: