Yep. I was going to recommend that one.
Its got a reasonably simple UI that looks very much like a low end CRO, and has X-Y mode, a spectrum analyser, and can concurrently use the sound card as a signal generator.
Note: its *NOT* freeware, but the free licence explicitly permits your intended usage
* The use of the software and of the documentation is granted free of charge for private and non-commercial use in educational institutions
* Any commercial application requires a corresponding license (see below)
* Distribution and sale of the program is prohibited. Contact the author in order to obtain a resale license.
N.B. any soundcard scope is *NOT* DC coupled. Although its possible to hack a USB soundcard to remove the coupling caps and add OPAMP level shifters (+ generate a negative rail) to make it DC coupled, that's a lot of work when you can get a 2 channel usb scope for as little as $30.
For showing sensors without confusion, you may actually be better off mounting a webcam on a selfie stick to the top of your usual bench scope. It will certainly help you *not* to be fumbling with unfamiliar scope controls during the presentation.