Although Huygens' construction isn't officially on the syllabus I always like to use it to show why light diffracts through a small opening. Today I found a great way to show how a wavefront can be made of a large number of wavelet sources using Paul Falstad's ripple tank. First remove all wave source and stop the animation, then choose edit wave and place dots along a wavefront. Start animation and watch the wavelets form a wavefront. If you have any Dutch students in your class you can get them to pronounce Huygens properly but don't try yourself.