Hello Rain,
Too many old machines have something that is hard to shift, if not jammed, so it would be useful to show that the slide plates, stitch length levers etc. move as they should.
Put on a seam guide, cut two nice straight edged pieces of plain coloured cotton poplin or heavier, and do a quarter inch seam (like on my seam guide video). Many people do not realise how easy it is to do a quarter inch seam without using a special foot.
Finally, use different coloured threads of exactly the same weight above and below. Then iron it before you do a close-up of the finished stitch. People should just about be able to see the thread from the other side where it twists for the lock stitch, but at the same time they should be able tell that there are none of the dreaded loops which you get with bad tension. Do a close up of both sides so they know that the stitch is equally good top and bottom.
And do a really fast commentary please in your best Officer Dibble voice so I can really enjoy it.
Best of luck!