If you are like any other Hobbytown, you sell model cars, trains, and remote controlled vehicles. That being said, LED projects work for all of those categories. You should make an effort to promote use of LEDs and the Arduino to get customers acquainted with Arduino. Perhaps you can have printed examples set-up with required parts and tools, those things will take some of the guessing out of the process.
Add that with some IR sensors to detect trains inside of a tunnel or even calculate speed. I heard that RadioShack is having posters "Weekend projects" with arduino. Have a look and see what you can do in terms of bringing people into electronics and lowering the initial difficulty.