People are always looking to have their cars washed and detailed. You could be a mobile car washer and detailer with a permanent location. Reach out to people you know or make some flyers and put it in your neighbors' mailboxes. If you want to get serious about it, prop up a one-page website or give out business cards. You can make money quickly doing this.
Whether it’s an important consumer application, a specialist app to solve a particular niche problem, or even a time-wasting game you can play on your phone, you can create a massively successful business if you build software that helps people. (Look at the rise of Slack—the team communication software that went from side project to billion-dollar company in just 2 years.)