Programming

As the other person said, Python. Or if you want something lower level, how about Rust?

I'm gonna be the weirdo recommending Ruby. I really like it as a language, and it's pretty fun to write. Plus it's got a lot of libraries so you can do fun projects like Discord chat bots.

Odin is quite a fun new language I just started learning. It is meant as a C replacement and comes with a decent standard library and third party library so there is a lot already built that you can use. It also is fully compatible with C and can use C libraries.

Just be warned that documentation is lacking and you will have to read the source code of the standard library from time to time or seek help from their discord.

Download GODOT and learn the scripting language and the shader language. Make a game! Or at least neat shader art.

Bonus answer: Csound. Make funky noises with code.

Clojure, it's a lisp that runs on the jvm and would be quite the shakeup for ya.

Go. It'll be just different enough from what you have experience with to make you think about things differently (in a good way!) from now on. And it's also a fantastically well-designed language that's great for getting real work done. And it's lightning fast as languages go, and compiles to an actual executable. Really a pleasure to work with. It's my (no pun intended) go-to language for every new project I start. (Excluding what I write specifically for a paycheck. I don't have a choice there.)

These come to mind:

• Elixir to expand the way you think about problems (and maybe your career).
• D to bring familiar conveniences closer to the metal.
• C to understand (and maybe contribute to) a vast ocean of existing software.
• Python for development productivity.

Alternatives

• C#, because it's good
• Rust, because it's different
• a (purely) functional language, because it's different
• Nushell, nice shell, and great for scripted data processing

Kotlin would be the most straightforward/useful professionally if you're primarily Java. Other Java stack options are Scala or (shudders) Clojure

C# has been doing a lot of really cool things lately, and has first-class Linyx support.

I love Clojure+Clojurescript

Have you ever wanted to do more with regular expressions? Then give Perl a try.

C

I say that you should find some interesting project, possibly something related to some desktop environment like Gnome, KDE, sway, cosmic and so on. There are multiple fun/interesting projects around them. Then pick a small and manageable task, use that to learn the language that project uses.

I find Cosmic to be a very interesting desktop project, and they use Rust if that would be of interest.

While it's related to your job and not very new compared to your current stack, it's very worth it to learn typescript. It has a cool type system and makes frontend development sane.

DotNet is closest to Java, but hang on to your hat: the state of C# is at least half a decade ahead of Java, if not a full decade. It’s sophistication will make Java use feel like banging rocks together. DotNet Core can now run on all three primary platforms, and with some careful work, you can write a single program that can compile down to each platform and carry along its own required binaries, no pre-install of any framework needed.

My second recommendation would be Rust. Stupidly steep learning curve, but an absolutely game-changing one where safety and security is concerned. It’s my next objective, personally speaking.

Any other language I could recommend starts getting into speciality purposes, which makes general use more difficult or even wholly inappropriate.

For example, if you are dropping into DotNet for business applications, I would also recommend diving into F# for that functional goodness for building complex business rules and data handling. But building an entire app in F# can be jamming a round peg into a square hole under many circumstances, it’s appropriateness envelope does not cover as many cases as C# does. You want to use C# for boilerplate/frameworks, F# for the core bits where it is going to shine.

Factor!

It's incredible and elegant and defies some common categorization.

I've put some of my favorite resources in the sidebar of https://programming.dev/c/concatenative and I'm happy to walk through any particular challenges/examples -- I've done about the first week of Advent of Code with it this year, and the most recent handful of Perl Weekly Challenges, and some basic Euler problems.