The thing about cardboard is all those corrugated boxes you plan on re-purposing are not identical in thickness or material. This means you’ll probably use settings that will cut through everything. The other thing about cardboard is that if you use more power than absolutely necessary (refer to previous sentence) the edges are sooty. You now have people with black, smoky, powder all over their fingers. The other option is to under power the cuts and at best you have perforated cardboard. If your open house is mainly for current and former girl scouts it’ll remind them of smores and you’ll be good. I guess it depends on your crowd.
Ideally, but it doesn’t sound like you have time, is to buy a bunch of cheap white ceramic tiles and a bunch of sharpies. You could cut a jig from your cardboard, secure the jig to the crumb tray and let people engrave tiles and then color them. See this thread for what can be done.
Having been part of many a product demo, and demo fails, it sounds to me that the time just isn’t there. Next Thursday sure, but tomorrow, I think it’ll be too rushed. If you have a bunch of proofgrade draft board you could cut out the gift of good measure project. People really like pushing the pulsing blue button and then seeing the laser head move around. The first time you see a laser cutter in action, that part is pretty freakin’ cool.