Update 2015/11/01: The Yuotube video was broken halfway through so I've uploaded a new one.
I wanted to make an interactive Jack-O-Lantern that would change colors when people get near it. I might add sound effects as well if I have time before the holiday... It uses about 200mA of a USB battery pack per hour. (You can alternatively hard-wire the power to any 5VDC supply.)
The system itself is simple and only consists of a Rainbowduino LED controller, an 8x8 RGB LED matrix, an HC-SR501 PIR sensor and a battery. The lighting pattern is controlled by the PIR sensor. By default the LEDs emulate a candle until motion is sensed at which point it flashes pseudo-random colors. There are a couple other color patterns in the sketch as well. All of them are just examples of what can be done.