If you live in an area where Uber or Lyft operate, why not become a driver? If you're looking to make some short-term cash, you can definitely rake it in by working for one of these popular car-hire apps. As long as your vehicle fits within the specifications of their program, and you have a clean license, you could do this on the side, especially if you're in a crunch for cash.
Upwork: This website offers a great marketplace for selling just about any professional service. You don't need a merchant account, website of your own or anything else for that matter. All you need to do is be able to provide a high-quality service at a reasonable price. But be informed, you will have to compete with many others that are constantly bidding on open jobs.
Are you seeing the possibilities here? Think about what book, or series of books you could write to help people in your field of expertise? You might start with an eBook offered on your own website that covers the basics. Use the eBook as an opportunity to tell people about your more in-depth book on Amazon and finally, follow through with an advanced book offering.
Start a photography business and sell your 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.