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Depends, define free and define useful. ;)
You can learn a lot by reading up and watching YT vids. I already did my own car maintenance (started as hobby, now it's a necessity, car mechanics cost twice/trice my hourly wage), but didn't know how to replace the fuelpump on this modern car. Garage quoted €500, part was 109, special tool 79, seals 5 each (2). Replaced it myself in 4h.
I'm also learning to play bass guitar with the help of YT (already have the bass), painted my house, replaced kitchen and parts in bathroom including plumbing,... None of DIY things for free, but cheaper then hiring somebody to do these task. Even when you count your hourly wage, most of those tasks can be done cheaper yourself.
When you want no investment at all, languages are very useful to learn, at least when you meet others speaking those languages. Learning to setup a computer with free software is usefull as well (saves cash, so eeven better then free ;) ).
At this moment in time, in most pasrts of Europe and the USA, labour is the most expensive part of everything, so I'd say everything you can learn to do yourself is worth investing some time and maybe some cash in. Nothing wrong with hobbies that can save cash in the long run.