Via Talk Like a Physicist, an ad campaign by Science World in Vancouver BC. (and I have a vague recollection of visiting it back when I lived slaved there as a postdoc)
I can’t find any link to this at the Science World web site but then this is apparently from a little while ago (other ads I found are dated 2006). So I can’t verify what was said in the pamphlet that was supposed to be included with the scale, because, of course, you only “weigh less” as long as the elevator is accelerating down. After it reaches “cruising speed” you weigh the same, and when it accelerates in the upward direction to bring you to rest, you “weigh” more, all happening on the way down. This reminds us that the scale is measuring the normal force, and not the weight, which is what I hope the pamphlet pointed out.
Another way to test this is to jump right as the accelerator starts down. In the ones that really zip, you can hit your head on the ceiling as the elevator accelerates while you are in a ballistic trajectory.
Good demo meets bad copy writing.
The demo I always wanted to build would consist of two devices: A balance to measure mass and a spring scale to measure force. The former would always give the same value, proving that your weight in kg does not change at all. [Here I use weight as defined in common use for weights and measures, not as a force on a scale as used in physics.] In contrast, the force would change, which would demonstrate the equivalence principle. Maybe one of these days …