Courses: P3, P13
A series of standing tubes filled with water and connected to a pipe that has a constriction in the center. When there's no flow all the tubes show the same water level. When flow is initiated, the tubes at the constriction show a lower level -- lower pressure -- a la Bernoulli.
Add a small amount of blue ink to the water to make it more visible in the tubes. This demo requires a sink for both a source of water and for a drain. It's conceivable it could be rigged to work with a couple of 5 gallon buckets as source and sink, but the source side would be tricky. One has to adjust the source flow rate to match the drainage rate and keep the overall water level steady.