Become a freelance photographer and sell photos online. If you have a nice DSLR camera and take good photos, do photography sessions or take photos of events, like parties and weddings. As another option, create fine art photos that people may want to hang on their walls, or take stock photos to sell online on sites like iStock Photo, Shutterstock, or Alamy.
Whether for high school students or adults, you can monetize your expertise by teaching people with less experience or knowledge than you in that subject area. You can work with an established group like Kaplan for, say, SAT tutoring, or you could try hanging out your own shingle and making your services known either to students, parents and schools in your community. If you are targeting adults, you can create your own website or list your services with adult tutoring companies.
If you’re strapped for cash but want to work on your own schedule, you might want to consider becoming a driver for Uber or Lyft. You can drive as much (or as little) as you want and set your own schedule. Plus, you get the added bonus of having interesting conversations along the way (or, at the very least, some fun stories to tell of your travels with strangers).