Create a simple oscillator using relays?
If you feed the coil via a normally closed contact of the relay, as soon as power is applied the relay will open-circuit its own power feed and turn on and off at possibly tens to hundreds of times per second.

This can be slowed down by putting a large capacitor across the coil; as soon as power is applied and the relay energizes, power bocomes lost but the capacitor is charged and holds the relay energized for a little bit more time. The bigger the cap the more it holds the relay energized and slows down the frequency of oscillation.