Yeah, this is probably my biggest hurdle with this. If I had to learn something for work, that'd be different, but I was thinking last night that mostly what I'd be doing is just learning new syntax.
You could learn an esoteric programming language, if that sort of thing strikes your fancy.
Velato, for example, is a language which uses MIDI files as source code.
the pattern of notes determines commands. Velato offers an unusual challenge to programmer-musicians: to compose a musical piece that, in addition to expressing their aims musically, fills the constraints necessary to compile to a working Velato program. Each song has a secret message: the program it determines when compiled as Velato.
Piet, on the other hand, uses blocks of colour as syntax, so that the end product -- the program -- looks like an abstract painting that might have been painted by Piet Mondrian.
I mentioned Uiua above, which certainly looks esoteric when used, at least, even if in actuality it's a "typical" stack-based code golfing language.
Or you could learn to use the
Wolfram Language, which is less a language (though it is that) than an interface to Wolfram's database of facts and ontologies.
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It's really a question of what you want to learn. Consider the following (thoroughly non-scientific and not exhaustive) pyramid of knowledge:
7. how to write good programs, how to write programs efficiently
6. frameworks, libraries, toolkits
5. programming languages
4. algorithms, data structures
3. operating systems, networking, databases
2. computer hardware
1. electrical engineering
0. classical and quantum physics
Concepts at layer n rely on those at layer n-1, and therefore indirectly on everything below themselves. (Also it's not really a pyramid, it's a graph; programming languages et al. obviously rely on theoretical computer science, and theoretical CS is a branch of mathematics and as such not limited by the constraints of our physical universe. But let's set all of this aside for the time being.)
So, consider which one is most appealing to you. Even "just learning new syntax" is educational and valuable, if the new syntax in question is sufficiently different from that which you already know. But you can also choose something farther afield.