Apprentice training exercise

I am an electrical engineering apprentice and was wondering if anyone could help me out,
i have designed and installed an apprentice test rig, for future apprentices, as part of a project. The ring includes motor testing, motor control and PLC however i am now looking to add just a general fault finding exercise. i am struggling to come up with a circuit idea that is not to easy but not to complicated, and only uses everyday components, such as, contactors, lights, relays, push buttons, sensors etc.
would really appreciate if somebody who has designed one before or has any ideas in mind to let me know.
thanks,
Lewis