I would say learning about any form of computing not related to Unix. Many of us old-timers grew up using systems (both home and corporate) that weren't variati

I would say learning about any form of computing not related to Unix. Many of us old-timers grew up using systems (both home and corporate) that weren't variations of Unix, and I feel like CS education is turning into a Unix mono-culture these days. User interfaces from things like VAX, Apple ][, IBM Mainframes, etc. The file system on macOS 8/9 which didn't use paths and has always allowed spaces in names because it isn't interacted with via command line, for example. The VAX file system that automatically versioned documents without the user needing to worry about it. (I think the Apple Lisa did something like this, too.) Don't get me wrong - there's a lot of bad ideas in those systems, too. (Like why do I have to use "RUN" vs. "BRUN" on the Apple ][ depending on what type of program it is?) But finding and promoting the better ideas and teaching young people about them is, I think , important and risks being lost otherwise. Learning about failures from older systems is always interesting, too. Why couldn't Atari and Amiga compete against Apple and Microsoft, for example? I think things like that are important to understand going forward, too.