Degrowth

Learning to sew and craft cloth and leather pieces with minimal spend on thrifted tools is absolutely worth it. Self-sufficiency, creativity, even being able to craft for others at a profit. The only investment outside of the initial start is patience!

This is a really cool question!

If you're looking for advice specifically on cosplay, I'd second sewing, tailoring, and thrifting, as well as picking your characters based on what you can do with the materials you have/what you can get used. People can be wonderfully clever when they're looking at the resources they have and figuring out what they can do with it. Also: Buy Nothing and Everything is Free groups might be able to help a bit if you have time (getting something used/for free tends to have a tradeoff in waiting for one to show up, but not always).

If you're looking for other hobbies that can be done with little waste, writing can be about as close to zero materials as it gets - your starting material is essentially other stories and your own lived experiences. You can do it on scrap paper or basically any laptop that still turns on (my main writing computer at the moment is an old Chromebook with two gigs of RAM running Alpine Linux with Wavemaker Cards for its writing software).

Drawing, especially the fundamentals, can also be practiced with basic materials, I did a lot of the anatomy drills with a mechanical pencil on computer paper. And if you get into digital art, depending on what tools you want, a laptop and mouse might be enough (that's how I do my photobashes, though I do also have a drawing monitor which I use in some of them).

Another option is to look at what resources you already have access to (know someone with tools? Have the means to print for free?) and see if there are any hobbies that interest you that those resources would allow you to do. Often tools and such are as big a part of the consumption as the materials.

Poetry!

Crochet for sure, making blankets and clothing for people in need and just giving it to them is probably anti consumer

There are some awesome hobbies that involve no consumerism, but wanted to give a shout out for making existing consumerist hobbies more sustainable:

• With cosplay- try making more costumes from existing clothes/costumes rather than buying everything new. Even picking themes that let you reuse stuff hekps reduce waste.
• You can often loan things, if you like reading you don't have to buy new books for everything! Libraries are a great resource and often stock new bestsellers.
• When you can't loan, second hand items are also great. Fast fashion is particularly unsustainable, but buying second hand clothing is a lot more sustainable.