I don't know if you would consider it "income", but in the spirit of hacking I am an active travel hacker and have accumulated millions of miles and points over the past 3 years or so through a combination of credit card bonuses, strategic spending, and "manufactured spending."

I consider miles a valuable currency at this point in my life because I would otherwise spend a lot of real money on personal travel. I can't remember the last time I paid for a flight and can easily accumulate dozens of free hotel nights a year.

Worth looking into if you value travel.