Reverse-engineering is a a useful exercise because it involves analyzing thousands of lines of
undocumented code and determining what it does. My experience involves contributing to Magic Lantern,
hacking Ambarella cameras, and starting the Fujihack project,
as well as reversing vendor network protocols, such as PTP.
Hardware:
Repairing technology and selling it (laptops, tablets, phones)
Resell/restore things locally, and sell on OfferUp/Mercari/FB Marketplace/Nextdoor
DIY Mechanic, work on riding/push mowers and older cars