Definitely show the charging voltage on a capacitor. Maybe a 1k resistor feeding a 0.1 ufd capacitor to ground. Feed it with a signal generator or even a 555 timer set at an appropriate frequency. Turn the probe around and watch the current waveform by probing across the resistor (as long as everything is floating). Or change the circuit such that the input goes to the capacitor and the series resistor goes to ground (differentiator circuit).
As much as I like your idea, I would advise against any measurement where the ground lead is not connected to ground. Sure, it works when the circuit is floating but doing this will encourage the audience to do the same on other circuits: "it worked there, so it should work here, too". If they do it on a non-isolated circuit... yeah, I'd rather not. Best to avoid showing them cool tricks that could get them into trouble because they don't understand when it works and when it doesn't.
While it may be simplifying the facts, it may make sense for techy101 to tell the audience that the ground lead must always be connected to ground, meaning that each probe can only measure a voltage relative to ground. Sure, it's not the whole truth, but it's not wrong either and prevents them from blowing up stuff.
I really, really like the idea to show contact bounce (any simple switch or button will do, ideally with a connected logic gate on a different channel to correlate), to show voltage across an RC filter (as above), overshoot/undershoot/ringing on wrongly terminated logic lines (nice way to introduce transmission line theory, which would allow you to seamlessly explain why a short ground spring is preferable to the ground lead), examining an Arduino's power supply noise when a DC motor runs from the same (allowing you to make mention that this is why those supplies are usually separate), etc.
Essentially, I suggest you choose subjects that aren't too difficult to understand while still applicable to electronics and the Arduino world in particular.
Also, I'd like to note that it's not always about the circuit - it's also learning how to operate the scope to make measurements (by eye or by cursor). Even a task as simple as determining a signal's frequency by eye may prove to be a challenge for some. I'd say that that's a basic skill, though, so it should be part of the training. It also makes the audience more familiar with the controls of the scope (V/div settings, horizontal offset, vertical offset, etc.).
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