Regardless if you need to earn some fast cash or we're just talking about making money in the grand scheme of things, there's an important psychology that needs to be mentioned before getting into the strategies. If you study Freud's model of the mind, you'll discover the Psychic Apparatus. It's the three-part construct in your mind that controls all of your behavior.
If you love animals and live in an urban area then walking dog is a viable side hustle idea. People are busy going to work each day and are forced to leave their animals stuck in the house. This is not good for the dog or the owner. Not only because dogs get lonely and need to be around people, but also because they deserve fresh air and the chance to go to the bathroom outside! Offering to walk a group of dogs once or twice a day can really rake in some extra money. You could create signs and hang them up around libraries and churches, or perhaps take the modern approach and sign up with Rover. Rover is an increasingly popular marketplace that connects dog owners to dog walkers in their neighborhood. Check out Rover today.
With apps being constantly developed, competent app developers are in high demand. Again, if you have the experience or skills to create successful apps then this could be something you could end up working on as a full-time freelancer. Check out adverts for developers on Toptal, a higher paying jobs site for experienced freelancers wanting to make money online.