One of my favourite teaching aids, an old Kodak Retina 1a camera.
A great way to understand aperture and shutter speed is to look at "old fashioned" fully mechanical, fully manual cameras and lenses. Modern digital SLRs work on exactly the same principle as the simplest pinhole cameras to allow light in through a lens to expose a light sensitive recording medium, either film or an electronic sensor.  Seeing how the iris in a lens opens and closes and how the mechanical timer sets the duration of the shutter speed is a great way to understand exposure. The built in light meters of modern cameras make getting accurate exposures child's play.  

However, the thing about great images isn't necessarily technical accuracy, perfect exposure and pin sharp focus, but composition. Learning how to control exposure is the easy part; developing an eye for composition, a way of interpreting the world and depicting a scene, that's the tricky bit, and something I also cover as part of my workshops and personal tuition sessions.