I know you can mess around with fiver, and do random stuff for five bucks online. Some guy managed to achieve an income that started from an "I will record myself saying any sentence you want me to for five bucks." But for the most part it's more professional stuff.
That being said, it's not for everybody, myself included.
If you've got spare cash, investing it into stuff can be good. I know some who collect and trade coins. It doesn't have to be coins, it can be stocks or anything really.
If you want to do something, you could try to make something that you could eventually sell. Fermented vegetables like Kimchi and sauerkraut are big in health circles, as are kvass (apparently), kumbucha. Honestly, I can't stand any of it so I steer clear, but you could conceivably put anything into mason jars and sell it at farmer's markets. Same with wood crafts, if you're in HVAC you're the handy type compared to me, and the last time I got dragged through a local arts and crafts market I couldn't help but realize how easy the stuff would be to make yourself. A guy with your knowledge could definitely figure out how to make cribbage boards shaped like a race course or some piece of cheeky plywood cutout folk art or a fretless guitar made with a piece of oak and an old oil can. Sure, it's not guaranteed income, but if you've got a slow day you could definitely put time into making something like that and eventually put some of it out to market or sell to somebody who would go there anyways.