I've been noodling around with an ADS1115 differential ADC, piping the output into to the serial plotter tool in the Arduino IDE:

Using the Arduino UNO as a basic Data Acquisition system

I know you can get things like the DI-145 for $30, and that any good scope (actually any bad scope) would put this thing to shame, but the thing is, it was just fun to play with because the auto-scaling y-axis meant that you always saw something on the output.

That's worth a heck of allot if you are trying to motivate people in a classroom environment.