It really depends on your skillset... if you code or do creative work, building a theme, shooting stock photography, or creating music with low licensing fees are all good things.
Highly recommend against stock photography. Especially with sites like Pixabay, it's a race to the bottom. And when I sold stock photos ten years ago, it wasn't that profitable anyway.
I found this Inc article from January of this year pretty on point as well: http://www.inc.com/sujan-patel/you-don-t-need-to-be-a-millio...