SQL - understanding how to interact with databases will be an incredibly useful skill for whatever rabbit hole you next want to fall down.

FOSS, learning curve, and niche field? I got you. VLSI layout software or SDR (software defined radio) software.

VLSI layout is kind of software allows you to design electronics hardware at the silicone/transistor level. IOW you have to draw the layers of materials that form transistors (cross poly silicon and p/n-diffused silicon to form) and connect them to metal layers. Learn how NAND and NOR gates are actually formed. Make a 1 bit data storage (flip flop), half adder, full adder. Go as far as you want.

Example software:

• Magic (what I used in school): https://opencircuitdesign.com/magic/
• klayout: https://www.klayout.de/

SDR. Less intense, but also niche and gives you lots of related things to learn/branch in to.

Can be used to receive and decode radio/satellite transmissions. Normally people would buy hardware and use software locally, but some people make their receivers accessible on the internet (websdr) which can give you a first intro to it. After you capture data you can run it through other software to decode (depending on what it is). For example, you can capture raw weather image data from satellites and decode it to get the images that the satellite is transmitting as it's capturing the data.

A list of some SDRs you can access over the internet (doesn't require hardware purchase to use): http://websdr.org/

It probably won't be immediately obvious how to use either SDR or VLSI software but you asked for niche and learning curve, so enjoy. ;)

EDIT: Fixed some typos and grammar.

There's always software reverse engineering. As bad as it may sound to some people, the US NSA has a github repositroy for a tool known as Ghidra, under Apache 2.0 license, so you can look at the source code and determine whether or not it's spyware. If it's actually NSA quality, you know it should be good enough for reverse engineering old software that won't land you in trouble if found out by the original devs. Last update was 2 months ago.

I tried a little bit but have yet to go very far at all with it since I am an idiot who thinks he's smarter than he really is.

Though, taking a page from your book, I am interested in looking up rekordbox now.

ssh and scp (secure connection and file transfer)

grep (the standard command-line search tool)

sed (Stream EDitor, good for find-replaces once you learn how to use it)

awk (can operate on text files or command output that's organized in columns)

sc (terminal-based spreadsheet)

trash (safer replacement for rm that moves things to your desktop's trash directory instead of deleting immediately)

One week of keybr.com to improve touch typing skills, which should serve you across all your IT endeavours... Unless you choose to learn gimp and drawing stuff :P

Factorio

Ardour, it's a FOSS Digital Audio Workstation

https://ardour.org/

You could get JOSM and become an OpenStreetMap editor. https://josm.openstreetmap.de/

Also install gentoo.

https://buttplug.io/

Mmkay thank you, this is such a great thing to ask.

Sketching 3D in FreeCAD.

Blender and buy a 3D printer afterwards

Lilypond. Its music software. Learning it will keep you occupied for at least a week.

Emacs

You can do really neat thinga in QGIS -- like make your own professional quality maps, make map animated gifs, etc

There are a lot of newer node-based 2D editors (covering both vector and raster) like Graphite and PixiEditor. I haven't tried these... still wondering if any could easily do stuff like SDF textures or simpler mesh creation (2D workflow, part/face vertex color, maybe 3D via extrude/revolve).

There's also Wick Editor for a potential spiritual successor to Flash.

I don't have any suggestions, I just want you to know that you're delightful

some suggestions for your consideration in order of empowerment and learning curve first,

• AWK
• ffmpeg (~ & imagemagick ?)
• bash, zsh, or fish
• kew (it's just a really simple nice tui music player, no fuss). (... "Am I the only fun one in Digit's list here?" -- kew)

n_n

Some other things that sprang to mind, (generally tending to more gui things, some distro/packagemenent things)... audacity/tenacity, fontforge, gimp, gentoo and portage, bedrocklinux and brl, peertube self hosting, forgefed self hosting, suckless.org's tools, CRUX or KISS/Carbs, just playing around in stellarium, playing around in pioneer space sim, learning how to evade humanity's detection as an ai in the game called singularity, ;3

Oh... Here's an idea for you... Learn a text editor.

Think it's not fun?

Depends on which text editor you pick, and how much the fun suits you. ... Emacs has so many things (including games, like tetris and doctor). Even neovim has some extra things (~ opencode runs on neovim... tho it's not the same, lol). Or maybe you want something simpler, like nano, mcedit, or fin (I wrote fin, don't use it, it's scruffy... and only able to do that from learning some of the things mentioned, and more).

Yay for learning. Yay for asking for what other fun things to learn. Good post. Good question. ... Sorry my answers were not better. Heh. Caught off guard, answered just what first sprang to mind. n_n Doubtless many other more fun things exist.

vi

Blender

FreeCAD

I'd say GIMP for photo editing would be a good one to learn. It's come in handy in a pinch both at home and at work when I've needed something done up real quick.

Stage lighting controls are extremely fun to learn. DMX, ArtNet, etc. QLC+ is free and open source, there are TON of other ones as well (like controlling big stage lights with a Wii controller so they move like you).

There are simulators, but the prettiest I've seen so far is https://github.com/AcChosen/VR-Stage-Lighting. Despite its name, you don't need vr for it but if you do, you can watch your lightshow as if it was real!

Learn you a Haskell!

https://learnyouahaskell.github.io/

Voicemeeter potato. Enjoy learning how all your audio devices work, and how to add noice effects.

I know you said software, but I would like to suggest a musical instrument. They still require learning a new language and typically provide a huge level of self satisfaction. A cheap keyboard or guitar can provide literally thousands of hours of study.

Learn the basics of a couple of FOSS audio-software staples.

LMMS - tracker

Audacity - Audio recorder