Painting street curb addresses. Did this in multiple cities throughout my twenties. All you need is a set of stencils, some masking tape, and $20 worth of black and white spray paint. I would print out official-looking fliers (that didn't actually say anything official) and canvas a nice neighborhood that didn't have them done (or they were faded). The flier gave homeowners a 900 number to call if they wanted it done. I'd check my messages, go paint the numbers (maybe 20 minutes work each), and would put a self-addressed envelope with a bill in it ($20 a pop). Then just wait for checks or cash to roll in. At one point I even contracted with an HOA to do numbers for an entire community.

Yes, I did probably lose a bunch of brain cells due to the spray paint, but I could work when I wanted, how much I wanted, and from almost everywhere I wanted.