Everybody saw phong implemented. So how about:


water - there are tons of tutorials and it looks always great
shadow mapping - absolute basic in game dev. Multipass rendering is good thing to show. You can improve it with some kind of soft shadows (i highly recommend PCSS - easy effective or Variance Shadow Maps) 
bumpmapping 
parallax mapping - looks cool, and pretty easy if you got bump mapping done.
geometry shaders (if you do hairs/fur over the polygon - could be based on lines or billboards - they will love you :)) - whitepaper from nvidia
mirrors
post process - cartoon shader, old camera shader


glsl demo suggestions?
The example shown below (courtesy of David Eppstein) is a common example of a cubic graph  that admits no perfect matching:



Are there other examples of cubic graphs that do not admit a perfect matching and, unlike the above example, do not contain a vertex that lies at the intersection of three bridges (i.e. an edge whose removal increases the number of connected components in the graph)?
