Nothing you'd probably like?

Learn about birds

You could learn to sew. Make your own stuff and repair the stuff you already have.

Keep an eye on local marketplaces for vintage Singer sewing machines. They’re heavy and a lot of people are looking to get rid of them. Forget the people who think they have a gold mine because it’s an antique. Bide your time and research the strengths of different machines (two solid performers are the Model 15 and Model 66).

The vintage Singers and Whites are built like tanks. If they still move (no rusty barn finds), they can be calibrated. It usually just takes a little sewing machine oil. Familiarising yourself with how they work and what to look for will do wonders. Sewing is a great skill to have.

Roleplaying games. Download a free ruleset and a dice app for your phone and you're good to go.

Conlanging - the art of creating tongues. https://library.conlang.org/about/conlanger.html

Juggling.

Birding! Cornell's "Merlin" app is free and an excellent identification/logging tool.

I love going on hikes or nature walks. Some great free apps are Merlin (for identifying bird calls) and Seek (for identifying plants and animals). All completely free aside from getting to to trail, but even in city parks it can be pretty fun. A good pair of binoculars is a great addition if you have the $$.

Shoplifting can be both fun and profitable

disc golf. There is a small cost to entry, but you can buy some used discs for a basic starter pack to get going. From there just find a local course and start playing! However if you enjoy it (like I did) I cannot guarantee that it will stay a cheap hobby.

Collecting seeds

Research the shit out of something you are interested in. Go down all the rabbit holes you can find. Compile it into a simpler, better understood synthesis. Convert it into a YouTube vid and deploy. I'd watch anything if well researched.

Geocaching

Do you have a working laptop or desktop? Learn to code. Actually code none of that vibe code bullshit. Making games is fun and rewarding and has an amazing community.

Im a big fan of calligraphy. It can be free using just a pencil and paper that you have. But it is more fun if you spend the $35 on marker paper and a calligraphy marker.

I like knitting and crochet, if you go on car boot sales you can often find a lot of balls of yarn almost free, and there are so much things you can do, for you or as gifts!

Running costs a pair of shoes every now and then and extra laundry detergent but otherwise free. Like any other hobby you absolutely can spend tons of money on it but it's not required.

Runkeeper is a free app where you can log down your runs but a casio watch with stopwatch is good enough if you know the distance.

Also, contrary to what a lot of people think you don't have to push yourself, you can just be easy running (chill jogging) and still make massive gains in endurance, speed, physical health and mental health. Depending on your starting point brisk walking can be sufficient to get started and then transition to "let me just jog a bit" when you feel like it and still have massive gains.

You can even combine it with other hobbies like audiobooks, music and/or podcasts.

Running feels to me like "cleaning the pipes" of the body that's relaxing and makes the remainder of the day a bit more enjoyable.

I have spent most of my life partner dancing. Almost every organized dance I've gone to has been volunteer run. So while it's not technically free, it's pretty easy to get in for free if you ask nicely and trade an hour of your time.