Ask HN: How do you come up with ideas for beer money?

I usually read a lot about "you should solve a problem" for SaaS apps or startups. But how do you come up with ideas for apps/website etc... if you want to just get 2/300$ / month with no other needs? It's all about little problems? Is affiliate marketing the only way or there's something else? Passive income is usually a myth, so I'm willing to invest some of my knowledge/work if it makes me reach this small goal.