Small ghost figure to be used with LEDs and a piezo element, driven by an ATtiny85 or whatever. The LEDs are connected to a PWM port, so that light effects are possible. The piezo element can be used as either input or output, so that the figure can respond to its surroundings. Total cost, including LEDs, wood, breadboard, piezo element and ATtiny, is somewhere around $10 / €7.

The original idea of this figure comes from http://www.littlerobotfriends.com. Those 'Little Robot Friends' are probably way more complex and versatile, using a full blown ATmega328. This one, of course, is much more fun to build and to program.