Early on, ROI on active income is >>>> passive income. Passive income will give you 10% annual returns max (let's not count crypto). So you need a lot of cash set aside before it's worth it.
Can you get from say, $50k to $55k income? Invest in that first. Buy books, pay for expensive courses, even some of the shadier ones. The average ROI for shady classes is often better than investing in a blue chip stock.
Until you're probably at some point where you can't go from $400k to $440k or so. And in some cases the cost isn't financial - the path forward becomes several steps backwards spiritually and morally. Then you stop and start investing in passive income.
Active income isn't just salaries though. I make more per year from hackathons than bonuses. Running a business is a form of active income, and at some point it's easier to transition to that.
It's hard to give a generic suggestion, because it's related to whatever you're plateauing on.
Obvious one is anything related to your job. If you do front end, then animations, architecture, etc. If it's back end, then algorithms, etc. There's plenty on sales, marketing, copywriting, and so on.
Or if you don't have a good job, then a bootcamp and such. Certs are more of a starting point for people with no experience but need to prove their competence. The AWS certs are all very safe bets.
A lot of people like to do free stuff on YouTube or articles only, but the downside is that most of these are half-assed. I've taught classes for years and writing good material requires a lot of effort. It takes about 3 hours of prep and iteration to teach a technical class for 1 hour - stuff can be further simplified, code can be improved and shared, new things updated, and so on.
A level above that is hiring a mentor. Which mentor again depends on what your plateau is on. Most aren't hired directly; you pay in labor and such. Universities are an institutional form of this, but there's plenty other options once you get there.
The most generic suggestion is MasterClass. Watch it past specific things like tennis and chess, and you'll see the principles. Kasparov doesn't open with openers, he begins with the principle that you should attack two pieces at once. A lot of people there have been doing things naturally that they don't even realize what they're doing wrong. One interesting example is David Mamet is one of the best dialogue writer of all time, but he can't explain what makes it good. But there's one by Aaron Sorkin who analyzes his idol Mamet, and he goes into it in a way that us regular people can understand.
One I saw is courses to become a certified phlebotomist. It takes a certification; they don't just take random people off the street. That means it pays reasonably well. The courses cost something like $500.
Note well: I have not independently verified any of this.
Can you get from say, $50k to $55k income? Invest in that first. Buy books, pay for expensive courses, even some of the shadier ones. The average ROI for shady classes is often better than investing in a blue chip stock.
Until you're probably at some point where you can't go from $400k to $440k or so. And in some cases the cost isn't financial - the path forward becomes several steps backwards spiritually and morally. Then you stop and start investing in passive income.
Active income isn't just salaries though. I make more per year from hackathons than bonuses. Running a business is a form of active income, and at some point it's easier to transition to that.