The water-driven waterwheels (al sawaai in Arabic) used in the Fayum Oasis to lift irrigation water into the old canals are much touted in any promotional literature about the area, but I'm a little more cautious about them. They are unique in Egypt (no other area provides the kind of fast-running streams needed to power them) and historically significant (they were introduced by Roman engineers, and the design has not been significantly altered since). Their setting, however, leaves a lot to be desired, and unless you find the wheels themselves to be interesting, you're going to be disappointed.
The most easily accessible and largest of the wheels still in operation are the four in the center of the city. I would recommend avoiding these, however -- the square is garbage-strewn and crowded, and the police are rude and intrusive. There are seven more, however, on the canal named Bahr Sanussi that runs north out of the city. You can follow the canal on foot or by car until the houses become less dense, and you'll come upon the wheels after 10 to 20 minutes (depending how often you get lost). They are big, black, and shiny, and their wood axles make a loud groaning noise as they turn. The setting, which is fairly pastoral, is also unfortunately squalid, and your enjoyment will depend largely on your tolerance for mud and being the center of attention among crowds of semi-naked teenage boys who come to frolic in the canal.