Okay, so I’m working on a list of “frugal hobbies” - I work with a lot of teens/early 20s ex-foster youth, and a lot of them complain that hobbies other than scrolling tiktok are expensive. I’d love to hear what your suggested frugal hobbies are, or frugal hacks for expensive hobbies (knitting/crocheting, gardening, sports),
What I have thus far
To add onto Urban hiking, is geocaching still a thing? I know some of us olds still play Pokemon Go as well which I think can be free and get you outside?
I just read an article (NYT?) that the exercise gap between men and women starts in highschool when boys keep playing sports and exercise for fun but girls start to focus on body image rather than the enjoyment of something. I wonder if there are cheap fitness hobbies that someone can get into without needing much gear?
Singing, solo or in community groups, most are free
Crossword puzzles (most sites have the current day’s for free)
Baking cookies? I loved baking treats for my friends when I was in high school.
My library also has tons of classes of all different types (and many specifically marketed for teens), and they also have a free makerspace
Podcasts! (making or listening)
Puzzles
Chess
Some art supplies can get expensive but drawing at least is dirt cheap, and there’s lots of classes etc online.
A couple hacks for yarn crafts:
Put out a call for free/cheap tools and supplies among different circles, including local craft clubs and online places like Freecycle. Often folks are happy to support a newbie this way.
Cultivate friendships with others who do the same or adjacent hobbies, online or in person. People often share news of sales, coupon codes, etc.
Sometimes yarn and tools shows up at thrift stores. Also check online sources like Craigslist or whatever is active in your area.
Sometimes a knitted or crocheted thing (from a thrift store?) can be unraveled and remade into something new, but beware the sweater with sewn side seams…that will yield a bunch of short pieces.
Plarn: plastic yarn. Homemade yarn from shopping bags.
Ravelry.com for patterns and chatting about patterns, projects, yarn, etc. Talking about yarn is part of the hobby!
Library for otherwise expensive pattern books.
Local frugal hack for all things arts and crafts: the Scrap Creative Reuse store. All kinds of donated, affordable arts/office supplies, fabrics, crafting materials, etc.
Sewing and yarn craft are cheap if you use repurpised or thrifted materials. Eg making stuffies out of old clothes or crochet rugs out of t-shirts. Supplies like needles, thread, hooks, glue are quite cheap.
There may be makerspaces or libraries that have machines to use, or neighbors on BN willing to loan them, if they get to a point of wanting to level up and can’t afford one yet.
ETA LOL two people beat me to the punch while I was typing.
Hand sewing and basic mending - you can get starter kits with thread/needles/mini scissors at most chain grocery stores, watch videos & get technique books at the library, and for ongoing supplies (thread, needles, pins) there’s picking up old stashes from other crafters or waiting for sales for thread/notions. Also just generally helpful to make clothes last longer/fit better on a budget.
Libraries also often have access to:
Also often if volunteering with a Friends of the Library org, if they do book sales the volunteers often get first dibs on stuff coming in.
Writing is a cheap hobby! And with fanfic and local writer’s groups can be very community building too.
Cooking probably qualifies. It’s already something you have to do every day or few days, unless living in frozen foods and hot pockets. With some time and creativity (and online recipes) you can transform basic ingredients to great flavors and fancy plates.
Yay, I didn’t know Fort Collins had a second-hand craft supply store / makerspace, but now I found it! Thanks for reminding me these are a thing 
Woodworking can be very cheap with simple hand tools for cutting/joinery, most can be found second hand without too much issue. You can build a lot of functional stuff with a handsaw, drill, dowels, and hammer. Wood is the most expensive part but can be salvaged (pallets, retail/curb waste) and sourced from your local woods from recently fallen stuff.
Fishing, similarly, can be cheap if you have access to public bodies of water. Cheap pole + line + hooks + lures can be had for $20 or less and will be 80% as functional as stuff 10x more expensive. Lots of fishing gear can be hand made as well, there are a lot of online resources for people making their own lures and poles. Honestly my favorite part of fishing is just fish identification and observation which I guess could be a frugal hobby without the ethical considerations altogether.
Our fishing permits are so expensive and so hard to get here!
Gosh of course I took for granted the most expensive thing depending on geography. I know a lot of states do not require licenses/permits for children under the age of 12 but obviously that doesn’t help you here 
sketching
biking/skateboarding/rollerblading/scooters
zines/blogs/podcast
music/theatre
woodworking/tinkering/makerspace
coding (opensource projects, apps)
I forgot, the “rainbow girls” here do beadwork and many of the beads are donated (some by me). Ask girl scouts, other groups what they do and where they get their supplies?
growing flowers! sure, you can go nuts but you don’t have to. seeds are cheap and sunshine is free and thrift stores are FULL of cheap vases.
Adding on to this excellent post: Use thin yarn or thread. It will make a project take much longer but usually with very little increase in cost, so you get more craft time per dollar.
If there’s a local free skool, that’s a great way to take classes. Similarly, a bike church or co-op will often help folks learn bike repair and building for free or low cost.
Birdwatching is free if you don’t care about taking photos etc. Going for long walks is free.
Tool libraries are a great way to borrow tools for small amounts of money when you need them