An astable multivibrator circuit is simple, yet educational and also entertaining. The link http://en.wikipedia.org/wiki/Astable
has a schematic of such a circuit. Besides the transistors, resistors and capaciators, include a pair of LED's (light emitting diodes) between resistors 1 and 4 and the positive rail (the horizontal line at the top of the schematic). The easiest way to construct this circuit is to use a "breadboard". It is a piece of plastic with several hundred holes running halfway through. Wires and the leads of electronic components are pushed down into the holes to construct test circuits. A 9 volt transistor battery can be used as a power source. Most of the hardware needed can be purchased from an electronics parts retailer like Radio Shack. Once correctly wired, the two LED's will take turns flashing on and off. The timing depends on the choice of capaciator. The larger the microfarad rating on the capaciator, the longer the period between flashing.