Audio transcribing. It's not a well paying field, and I imagine it will be dying out soon. But you can find many microjobs willing to pay upwards of 6+ per hours of audio transcribed for newcomers. You can easily make more, but you get what you put in to it. It's not really difficult, but it's time consuming. And generally not going to replace your income unless you, like mentioned before, get a job doing this.
But it's a pretty simple way to make some extra money.