I wouldn't be suggesting ideas and approaches. I would instead concentrate on "few hundred dollars per month". I think you are setting the bar too low if you are in US. Let say this side project will generate $200/month and you are spending 15 hours a month on it. It means that your hourly rate will be $13 (which is way-way below software engineer hourly wage). If the goal is do something useful, then work on opensource (for free). If the goal is to get some additional income, then you need to find an idea which can grow more than couple hundred bucks a month.