Any oscilloscope have a square signal generator for probe calibration. That signal should be enough for teaching the basics.
I'll add a vote for a function generator. A square wave by itself does not really allow visualization of varying trigger levels. You need something slower than a square wave's edges for that, like a sine or triangle/ramp. Then you can literally watch the waveform "move" as the trigger level is varied. This makes the behavior of the trigger instantly intuitive. IMHO the trigger is one of the most powerful aspects of a scope, and it's important for newbies to really grok what is happening.
Another handy signal source is any sort of analog video camera, as is found in (far too many?) devices these days. Almost all scopes have H and V triggering which handles holdoff, allowing a single line or frame to be easily displayed. Zooming in and out (with the timebase) while triggered makes the horizontal sweep timing very clear, and the ability to wave your hand around in front of the camera and "see" the effect on the analog video data is another very intuitive demonstration. People can relate to video cameras since they're surrounded by them these days.
I've done the above in under 10 minutes with utter newbies - never touched a scope before - and they were then able to describe and demonstrate how a scope works to another newbie. Some of the terminology wasn't quite right, and they fumbled with the controls, but they *understood* what was happening and could demonstrate it given enough time. That's a win in my book for <10 minutes invested!