Basic induction with current visualisation
Though it is a classic, it has got a lot more effective with the introduction of rare earth magnets. You can investigate how the voltage produces is affected by where the magnet is moved, how fast it is moved, how strong the magnet is and how many turns the coil has.
To make the experience more intuitive this version visualises the flow of electricity with a flow of led lights.
A simpler version with just a meter.