My most favourite demo is "Freespin" which runs entirely on a floppy drive without a computer. Crazy stuff.
https://www.pouet.net/prod.php?which=53871
https://www.youtube.com/watch?v=36BPql6Nl_U
I'm more impressed by the eerie beauty of it than the technical achievement, even if the code was orders of magnitude larger it would still be wonderful.
Of course it's not the same, but "flying through a 3D grayscale orthogonal structure" is actually possible in 64
Apologies for the cursing, but that is the most bat shit insane demo I have ever seen. I'll just leave you with the top comment on the video: "Witches have been burnt for lesser sorcery"