'Tis the season to be jolly, so let's have some fun in this spirit. A fistful of components is all you need to get an alternating lighting effect. That is the simplest circuit I can remember and it is called astable multivibrator.
It is astable because it never rests in one state and it's called multivibrator due to it's spectrum of frequencies, beyond the scope of this post. Even for matched resistor and capacitor pairs, the circuit will begin to oscillate on its own. That's because perfect matching doesn't really exist. Either way it's fun to build but utterly boring to watch. Here's what you need:
Bill Of Materials:
Q1,2 - 2x 2N3904 NPN bipolar transistors
LED1,2 - 2x LEDs (Red and Green)
R1,2 - 2x 330 Ohm
R3,4 - 2x 10 kOhm
C1,2 - 2x 4.7 uF
You may be tempted to scale-up the circuit by adding LEDs in parallel and you'd be right. Be wary of the transistor's IC rating, 200 mA in this case, as parallel consumers increases the collector current.