Learning. Despite having a fairly good degree, I found the ability to learn through mediums like YouTube to be invaluable and free. I have access to more subjects and direct access to specialists in their field without having to be in a university theatre. The internet is beautiful at times.

Journaling. It’s very good for you too. Just drop by a dollar store and get a plain-lined book.

Cross stitching is very cheap and relatively easy to learn. I'm pretty fidgety so it's nice keeping my hands busy while watching or listening to something.

Sleeping I highly recommend

Music if you’re not a gear head.

Tabletop RPGs. You can spend a ton or as little as 15$ for infinite fun.

Programming: Most people can learn to program for free. You won't be a good software developer but programming as a hobby itself is free and you can do loads of fun stuff.

Small-scale container gardening, highly recommend. Pick any decently sized food-safe plastic container, put some holes in it, buy some potting mix, fertilizer, and tomato seeds, and that’s it. It's such a fun hobby and I learned an enormous amount of stuff about botany, modern agriculture, and plant diseases.

Disc golf. 20 bucks for a starter set of a driver, mid and putter and many courses are free to play. You get outside and get exercise while you play a game.

Sewing. Very useful, too!

Geocaching!

Writing. It is the cheapest hobby in the whole world and does nothing but make you and others happy.

Meditation. Have a heartbeat? You can meditate. Just sit down somewhere quiet and allow yourself to not have to do anything for 5 minutes. Your mind will race; you will get distracted, that's okay. When you notice you are distracted, just focus again on your breathing. Repeat until the 5 minutes is up. Once you get used to it, extend the time period and allow yourself not to have to do anything for 10 minutes. Just keep bringing yourself back to your breath whenever you notice you were swept away by your thoughts again. No judgment, it's all good, just get back to your breath. Keep giving yourself more time whenever you feel comfortable. It will keep being challenging, but interesting things will start to happen in your mind, your life, your relationships if you stick with it.

Standup comedy. You can walk up to an open mic and get your name on the list and talk trash for absolutely nothing. Sure, some say you should spend enough to write your material down, but you don't have to Not saying this means you're good at it, but if you don't run a big bar tab, you can have fun, meet people and whatnot, and all for nothing more than the cost of showing up.

Citizen Science. Record the plants and animals around you, even if it’s stuff you find in your backyard or the local park, and share it online. Many sites connect with GBIF, which allows scientists to use the data in their research, and just learning more about the nature around you can be an interesting and enlightening experience. I’ve had some of my beewolf photos cited in a research paper, have been contacted about collecting seeds and mites for researchers specializing in those areas, and have been contacted about potentially finding a species new to science. In 2015, scientists recorded 30 new fly species from Los Angeles backyards, so even urban areas can yield interesting finds. On top of that, it can be a lot of fun to learn about the differences in things that look the same and the relationships between organisms. iNaturalist and eBird are both fairly large international sites but there are local ones as well. All it takes is a cell phone camera to get started - and sometimes not even that! - and I do highly recommend it.

I collect the rubber bands the postman drops and now I have a rubber band ball 18” across. All free, all mine.

Fixing stuff. Most things can be taken apart with a cheap screwdriver set. Grab something of yours that broke, pull it apart and try to figure out what's wrong. Researching how to pull things apart, how they work, and what causes them to break is free. If you fix it, you've just saved yourself money from having to replace it. Worst case scenario: you can't fix it and you're back where you started.

Learning knots. It's not 100% free if you actually make things (like bracelets, guitar straps, etc.), but it doesn't hurt to learn a few knots. Just buy some paracord, go on animatedknots.com or some other knot tutorial website (Youtube works too), and start tying.

Learning a foreign language. Lots of free apps and podcasts.

Origami. All you need is paper.

Arguing online

Reading. Library is free if you return books on time.

Hiking If you have a comfortable pair of walking shoes and the ability to get to a large park, walking is pretty cheap and relaxing

Exercise. You can do plenty of routines like crunches, squats, pushups, situps and jogging and not spend a cent beyond a decent pair of shoes for jogging in.

Cooking can be a pretty cheap hobby. I mean, you have to eat anyway, might as well enjoy the processes of making your own food.