

Lockpicking is fun and has helped me out a few times when I lost keys. There’s some cool clear locks you can get to help you understand how basic locks work.
Electronics and soldering are fun and useful, allowing you to repair and repurpose devices.
Amateur Radio is cool as hell, you can talk to people around your area or thousands of miles away, it dovetails nicely with electronics and soldering. One way to reduce cost of this hobby is making your own antennas etc. But you can get on the air in the US for $60, 35 for the license, a free exam, and 25 for a Baofeng GT-5R, which is FCC compliant and I’ve gotten good repeater contact at nearly 10 miles.
Programming is useful and basically free if you have access to a computer.
Physical activity of any kind is excellent, especially if you can combine it with one of the above listed activities. Radio lends itself to this with POTA and SOTA.
You can even combine the two with data modes, check out kiwisdr for a little taste of what you can see. Totally free and there’s a bunch of plugins that can interpret signals you’re seeing from these random SDRs.