I think most of the stuff I make falls into this category...
My favourites are probably these two:
- a tool I made called gif-to-ansi (https://dom111.github.io/gif-to-ansi/) which converts an animated GIF to a stream of ANSI escapes in a shell script.
I made this because I wanted to loop a GIF image in terminal when I mistyped a command to train me to do better typing.
- a 26 "language" polyglot (https://codegolf.stackexchange.com/q/209669/9365) where each starts with a different letter of the alphabet and prints out the alphabet less it's starting char (e.g. Bash prints out ACDEF...).
This was just a puzzle that nerd sniped me, I really enjoyed it though and even got to learn a little APL for it, as well as brush up on shells. I highly recommend using sites like this if you want to explore a languages depths, I used it to learn Perl for a previous job and still play around in Perl today!
I might be misremembering but isn't there a key combo ([Fn] + [Ctrl] + [F7] or something?) that enabled control selection and shows the underlines. I haven't used Mac OS for a while so I might be wrong!