The Christmas tree is made from foam board with copper tape used to connect a 2032 coin cell to different LEDs as it rolls down a ramp. The slope and length of the rails plus the tilt of the tree determine how fast the coin cell rolls and long the LEDs will be lit. Tilting the tree back allows the coin cell to "stick" to the foam board as it rolls and keeps it from falling off.
CR2032 coin cells start out at approximately 3.2 V and are sufficient to provide the forward voltage required of common LEDs. The internal resistance of the coin cells is such that current limiting resistors are not required.
The positive side of the coin cell also wraps around the sides and the bottom side will ride along the rails and the coin cell rolls down the ramp. This then is the positive rail. The negative side of the coin cells projects and bears against a strip of copper on the tree itself and forms a negative rail. The negative rail is interrupted from time to time which causes the LED to turn on and off.
And here it is working...