I learn about EVERYTHING because I'm curious and (usually) read about everything. I've also *done* a number of things, which is extremely important for reinforcing that knowledge.
Examples from my past:
Metalworking. Why? Building stuff. Ok, so you use metal and stuff for that. Cut it, drill it, screw it together. Bend it to shape. Want more? Braze it, weld it. Cast it.
Casting? Foundry. Metal melting. Oh, but then you need to think about alloys, because not everything is good for that. Metallurgy. Microstructure. Chemistry of metals. Corrosion resistance.
Lots of fascinating stuff just there.
Oh, but chemistry. Everything is made of chemicals. How the heck does that work? Inorganic chemistry, the study of "everything else": metals, salts, rocks. The Earth's crust is made of rocks. The oceans are made of water (and some salts, and some other things). Metals corrode to inorganic salts under the influence of air, water, and usually some salts. Batteries corrode metals intentionally to produce electricity. Electrolytic cells can create interesting chemicals, like strong reducing and oxidizing agents. Things that are used in pyrotechnics. Pyrotechnics create interesting colors and patterns using metals and salts. Organic chemistry is primarily the reactions of carbon with itself, hydrogen, oxygen and a few others, rarely bringing in the rest of the periodic table. Just from those atoms, it's incredibly rich (which, however, I didn't get much into, but there are some fantastic amateur organic chemists out there).
And electronics. Practically everything we do, every day, involves high technology on some level. What's inside that silicon chip? Chemicals. Fabrication is intensive with chemical engineering and physics. Cool stuff.
Of these, obviously, I've stuck with electronics the most, but all these are intimately connected; life is inherently multidisciplinary. Investigate something deeply enough and you will always find something different or new down there, and probably connected with many other things.
Oh, needless to say: math underpins much of this. Not necessarily actively, but my knowledge of calculus and complex analysis comes in handy for understanding and connecting many of these things. Signals, systems, functions, they all belong together.
Tim