Electronic toys are bought by a market that includes a large range of ages.  Teenagers will Most likely be entertained by electric cars or buggies -especially ones that respond in some way to their surroundings.  This project is based on capacitors being used as a very simple timing device.  It includes a potential divider in which a light dependant resistor is used and a relay.  The relay is operated by the low current obtained from the capacitors.

It would be just as much fun if a microphone was used  in place of the LDR but the original intention of this task was to start two dragsters off at the same time by firing a small portable electronic flash unit  at the LDRs.  The winner might be the dragster that has the best bearings, the straightest alignment of wheels / axles and best power-to-weight ratio.

The sensor circuit depends on a simple potential divider including a variable resistor (to allow adjustment for light levels) and an LDR to detect the light pulse that will set the buggy going.
The power for this circuit comes from two capacitors that are charged by placing a 9 volt battery in contact with two metal rods. (These can be seen at the front of the buggy).  Once charged and with the LDR in the 'dark' the buggy is ready to go.

The output from the sensor circuit trips a small relay that turns on the motor circuit - this circuit includes AA batteries in a holder with a motor connected together across the 'Normally Open' contacts of the relay. 

The project illustrates the need for greater power than the capacitors can supply on their own and for experimental purposes different batteries can be attached to check out the effects
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Finally the buggy can have a vacuum formed 'funny-car' top and will travel several meters quite rapidly until the capacitors are discharged and the relay shuts the motor circuit down.