I've been looking into installing air conditioning in the building that will eventually be my office and workshop. Local HVAC contractors gave me extremely large numbers as quotes. I've watched mini split installs and I'm pretty sure they're within DIY range for me. So I spent $10 and a couple of nights of reading, and took the EPA 608 type 2 certification exam today, so now (pending someone watching and approving a 20-minute video of me staring at a phone screen) I can legally install one myself for significantly cheaper.

I've also occasionally considered getting a ham radio license, solely because I like a bunch of adjacent electronics topics and it feels like the kind of thing I could be into. (So far, I haven't bothered because the vibes of the community around it don't feel right for me, but I haven't completely abandoned the idea.)

It occurs to me there are probably several more things like this that I'd never thought about. Small plane pilot's licenses are another, though those require formal training. But attempting to research this mostly returns lists of "you could be an Official Microsoft Certified Professional Excel User!". That's not what I mean.

What other useful licenses can one easily get without making it a whole career? "Useful" here means something like "having the piece of official paper grants privileges that simply learning the underlying knowledge doesn't".