A thick rope is attached to a motor driver at one end, while the other end goes over a low-friction pulley and is attached to a mass. Different standing waves can be formed by adjusting either the motor speed or the weight hanging from the rope. The pattern can be "frozen" with a strobe, even with a fair amount of illumination in the room.
In an alternate version, an elastic string is set up between two carts, and lit up with UV light. It's a little harder to set up, but quite dramatic.