By far the best investment you can make is to upgrade your earning potential. I run a software consultancy and I could easily employ someone who specialized in writing software tests or a release engineer at 7x your current wage (these are unglamorous but ultimately not had things). Making 7x per hour changes the income and wealth equation dramatically. You could literally self teach both of those areas in 6 months. Two books from the library, free gitlab account, fork 2 open source projects, implement test suite, begin writing tests. Start submitting pull requests upstream when you get good.

80% of software companies can benefit from work in those two areas and they provide more value than they cost so there’s effectively infinite work.

If that field doesn’t appeal to you I’d suggest starting a business. A recent study of people who earn more than a million a year (maybe posted here two weeks ago?) has shown that it’s primarily business owners. Typically in boring, but very established value areas, typically with a local monopoly. Things like car dealerships, auto repair. The article (which I can’t find right now) also mentioned “marketing research”? which baffles me.

A clever person could learn a lot in the library over a weekend about starting and operating a business, about choosing a good business to start (businesses without a lot of competition, but a solid customer base are good. I’d choose one that will still work in an economic downturn)

Beware the taste for the expensive lifestyle. It’s easy to creep by degrees to a lifestyle beyond your means. If your expenses exceed your income it doesn’t matter how much money you make, you’ll be in debt to you reverse it. I’ve lived both ways and can tell you low expense is the only way to go.