When teaching diffusion and osmosis, there are a variety of labs available that demonstrate these processes. This lab uses a shell-less egg to show how water passively moves across a membrane. It is messy, and requires delicate egg-handling, but it illustrates the point well. The egg shrinks in syrup (hypertonic) and swells in water (hypotonic).
The lab requires a few days for the water to move, so it’s best to begin as soon as you start the topic.