Playing around with polarisers, like those found in polarising sunglasses, provides a good introduction to polarisation. Some exciting results can already be obtained by stacking and rotating two or three polariser sheets.
Mechanical stresses inside a transparent piece of plastic (like a ruler) can be visualized by inserting the piece in between polarisers. This way, one can diagnose where this piece is most likely to break.
By stretching wrapping plastic and folding and stacking it, one can create a colorful piece of art that can only be fully appreciated by wearing polarising sunglasses or applying a sheet polariser.