Met a guy in an interview that had, "Slang for tourists," as an app. Pretty simple, made him a few hundred dollars per month, Android only.

Another option is, given Math / CS, self publish an ebook. I make a few hundred from fiction, and on a good month, enough to buy lunch or more via my non fiction tutorial style ebooks on Amazon and elsewhere.

Scrivener to create, Amazon KDP and Smashwords to distribute (plus own website, etc) and you're golden. A technical book can sell for $100+ bucks per month at ten copies, $10 each, and if it's enough to save somebody 1/2 hour of work, it'll probably do very, very well.