Speaking of Demos, I used to try and get a bunch of scene demos for my Amiga waaaay back in the day.
this was a pretty cool one and fit on an 880k floppy.
Most currently available demo I can think of is Detroit: Become Human on PS4.
You didn't specify platform, so not sure if you're looking for PC only or something. I've been playing PS4 mostly here lately for its exclusives like this one.