What is the best way to earn money outside your career?

You may have higher returns from investing in career advancement. I don't have solid answers on any of this, but my best guess is my highest-value career investments have an annualized return of 60% (all numbers after inflation), while being an angel investor has a return of 35% (and unless you're already rich/high income or work in finance, you're limited to investing $2200/year in America), owning an AirBnB and paying someone else to do the work has a return of 22% (but this is wayyyyy higher than regular landlording, so I may not have properly priced in risks such as the pandemic), and investing broadly in the highest-risk, highest-reward sections of the stock market has an 11.4% return.

Figuring out how to invest money into your career may be tricky. The obvious example is education. But anything that provides skill-building or networking opportunities is potentially worth spending money on. And networking in particular I tend to think about more broadly than most people. Real networking comes mostly from working on shared projects, not going to conferences or whatever. So create those opportunities, and be willing to spend money to do it.

Edit: Returns on the stock market overall average 6.96%. I will consider any of the investments I have listed as circumstances warrant. Main thing to keep in mind is you get higher return, on average, by taking higher risk. But not stupid risks.

So this isn't a full answer to your question, but one thing you should factor in when making your decision is how much time/energy do you have? Stocks and bonds are pretty great for that because they're passive income earners. If you already have a full-time job, then renovating property might be a huge time sink unless you're willing to shell out the cash for hiring someone to do it. Some of this also depends on what skills you have. If you're a programmer, doing some side projects/contract work might make you some decent cash. If you can turn a hobby that you would already do anyways into a profitable venture, that can save you on the time/energy cost.