With no money investment...hmmm, sweat equity then.
What I did before is WRITING. I submitted to magazines and talked to people if they need articles/copy. Most of the time, I cashed in. Now, I understand that this won't be totally feasible due to the massive capabilities of AI, but there's still hope. You just have to amp your REACH and Persuasive Power. Eventually, someone will need your skill.
Another thing you can do is getting paid by sharing valuable information. This is sweat equity as well - you need to do your research, find a market, and persuade your prospects. If you're an authority in a given niche, even better - you have a leg up in the game.
Or you can try grindy games with community aspects allowing rmt. They won't make you rich, and will take a long time of grinding. But hey, they good for beer money!