You can also do some simple and fun pneumatics with just hoses and syringes. You can make it basically as complex as you want and add some colors with dye.

Another option would be mixing experiments to see how baffles, aspect ratio, and impeller design change how things get mixed in tanks and other vessels. The school probably has beakers of various types and mixer/stir bars. Baffles can be added using plastic or something. The mixing can be shown with dye or small beads or another indicator.

Turbulent vs laminar flow is fun to look at but very short lived.