Programming started as a hobby for me as a teenager. I always "liked computers" so thought I'd give it a try. I never intended to make a career out of it because it seemed so hard at first, but over a decade later, I'm decently accomplished in my field and get paid bank for it.

As a hobby, it's fantastic. You can add in missing features to open source software you use (including the one I'm posting this to right now!). You can make your own little apps to fill niches you haven't found an existing program for. You can automate boring stuff from other work. You can make mods for certain types of video games. Or if you're really ambitious, you can even make a video game (but I gotta tell you, video games are hard and need much more than just programming -- I do not recommend making video games as a goal unless you've thought out just what that involves).

If you make a career out of it later, cool. But even if you don't, it's a fun and rewarding hobby that costs almost nothing. As long as you have a computer (preferably not a mobile phone, though it's technically possible to use a phone), you can program. Hardware doesn't generally matter. Any cheap laptop works. All the tools you need have free and often open source ones you can use. You only need to pay for web hosting if you make a web tool and want to share it with others.

It's not free, but you have to pay for it no matter what, so it kinda is, but cooking. Everyone should learn the basics of navigating a kitchen and the basic terminology.

Singing and dancing are both learnable free. Art in general tends to be cheaper. Usually anyway.

Intellectual skills are all fair game, there's wide availability online. You need to start with Information Literacy though, to kinda unlock the rest. Otherwise you'll waste too much time on bullshit.

Next time something breaks, try repairing it. Pull it apart and look for whats broken, google if you need info or ideas on fixing the problem, and go for it. Worst case scenario you can't fix something that wasn't working, best case scenario you've saved something from the scrap pile. Either way you'll have a little bit of knowledge you didn't already, maybe some skills

Touch typing.

Lear how to cook. Just grab a simple online recipe, a bottle of your beverage of choice and try cooking something. It's a really simple and useful skill acquire.

This is an animated site for teaching you how to tie different knots.

With a piece of string and perhaps a few wooden dowels, you can teach yourself a variety of useful knots for different needs.

This barely qualifies as a hobby, but at some point I decided to learn how to count in binary on my fingers. It's handy if you need to hold a number in your head for a bit and can't write things down, or to count past 10 visually on your fingers for somebody. There are probably YouTube videos on it. I literally can't remember where I learned, but I practiced a lot when bored in church. It's relatively non-disruptive and practicing can eat a decent amount of time.

0 materials. Just takes time. Literally free.

Clothing repair. Patch, darn, replace a zip or button. Expand that to full sewing and it isn’t free but you’ll save a bucket of money, have clothes that are exactly what you want, and don’t disintegrate in 6 months

I learned the basics of 3D printing and Fusion 360 (CAD software) via youtube. And the 3D printing has gotten easier since I learned.

Learn a new language. Tons of free resources online. Borrow books or audiobooks from your local library. Make friends with others interested in learning and teaching each other new languages.

programming

I just recently took up cross stitching. You can learn all the basics for free on YouTube and there are tons of cross stitching communities online and there are tons of free patterns online too. All the supplies you need at first are some thread, some Aida cloth and needles. When I started I got a stitching hoop too I think my total investment was about $20.

Mental math. People overcomplicate how difficult things are but quickly calculating numbers and percentages is a super useful skill that'll save you a lot of time.

If you have a patch of soil with good sun exposure you can learn how to grow your own food. It's not free but you can be very thrifty while doing it. Huw Richards has published a nice book on the cheapest ways to garden.

Learn a language. It's a great thing and you can easily do it instead of browsing social medias while traveling. I started with japanese about 5 months ago and can understand some phrases of native speakers already.

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.

Check out The Odin Project. It is a free full stack developer course. I'm going to give it a try myself.

You can teach yourself how to change the oil on your car simply by watching YouTube videos. And then all you need to buy are a few basic supplies, and you'll be able to change your oil for life. Oil changes are either absurdly expensive (dealership) or done half-assed (Jiffy Lube), so doing it yourself gives you a job done right for pennies on the dollar.

And if you find that you have a knack for it, you can teach yourself more basic maintenance jobs like changing brake pads.

Additional languages beyond your native tongue. Spanish, French, Chinese, American (in my case) Sign Language, etc. There are so many apps that teach even the most basic conversational skills in the most commonly spoken languages, and a lot of content can be found without upgrading to a premium version. It's a great bonus job skill, and an excellent way to make new acquaintances.

Cooking. Well, I guess it depends on what you mean by "free". I guess tree climbing or parkour. Or, languages too.

You can learn how to make a donut: https://youtu.be/nIoXOplUvAw