For me, it was being a firearms dealer at a pawn shop in the hood. I had been self-employed or working boring office jobs for a long time and decided I wanted a change of scenery and to learn something new. I took an hourly pay cut, but really enjoyed the job and made more money from just buying stuff from the shop and reselling it. I learned about pretty much everything. Guns, tools, fine watches, jewelry, negotiation, resale value, and the art of buying used items (which save you money and lets there be less waste).
I love hearing about niche industries that you learn a lot of life skills from, so tell me about yours!