If you're a practical joker or just like to have fun with children of all ages then the following project by the Instructables team will fit the bill. They've created an Arduino-powered "monster" which responds to changes in light levels to determine when to make itself know in a scary manner. It uses an inexpensive sound record/playback module for sound effects, and a neat enclosure with LED eyes helps with the effects.
By using the light sensor and possibly a motion sensor the monster could make various noises until the person nearby turns on the room light or moves about, possibly frustrating or scaring them even more. The system is demsonstrated in the following video:
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