The action of a diffraction grating itself can be demonstrated by passing a laser beam through the grating and showing the spread out spots on the wall. You can lead into the phenomenon by showing the effect of Single, Double, and Multiple Slits [1] with the Cornell plate. Interesting effects result from crossing two gratings in the laser beam, or by arranging many at all different angles.
Links:
[1] https://demoweb.physics.ucla.edu/node/90