A venturi tube is used to illustrate the changing pressure of a fluid through the change in speed of that fluid. This demonstrates the physics behind the design of an airplane wing and the science of heavier than air flight. Basically, a venturi tube is constructed from a PVC pipe that tapers in the middle as it connects two or more pipes of differing diameters. There is also a clear tube connecting the two sides of the taper. If this connecting tube is partly filled with water, you can watch it move as air is forced through the pipe. The moving water demonstrates the change in air pressure between the two pipes.
A didgeridoo is one of the oldest musical instruments ever made. It is customarily made from hollowed-out wood but can also be made with cardboard tubes or PVC pipe. The instrument is played by blowing into one end and creating a buzzing sound, similar to the way brass instruments are played. Alternatively, you could leave both ends of the tube open and tap on the side of the tube or either of the ends to create a sound. The physics behind these sounds and the air pressure changes used to create them can be found in the articles "Connections: Standing Waves and Wind Instruments" and "University of Illinois: The Physics of the Didgeridoo."
Air pressure can be stored in a PVC pipe and released using a valve connected to another section of pipe. The section of pipe that is not under pressure can contain a small rocket or other projectile that is propelled forward by the changing air pressure in the pipe. In this experiment, you can test differing pressures and the distance the projectile travels.
A variation on the rocket launcher is to try a variety of projectiles. One common option is to use a potato. This is because it is possible for a potato to be forced into a PVC pipe and create a strong seal that will allow the potato to be launched further than an object that does not seal well within the pipe. This experiment could test different projectiles having different sealing properties, each launched using the same pressure.