Ask HN: I have ~12hrs of free dev time a week. How do I turn that into money?

I'd like to hear people's opinions or experiences on what I could be doing in my spare dev time to earn money outside of my fulltime work. I'm a senior iOS developer with a well paying 9-5 job, however my wife has decided to leave her full-time job to pursue her career as an artist, and so I'd like to be able to makeup somewhat our drop in income.