Become a dog walker if you live in an area where they are in demand. You can walk dogs for people who own dogs but who are too busy to walk them. However, this can be a demanding job, especially if you end up walking a few dogs at once or if you are walking dogs throughout the day, so ensure that you have the physical stamina for this type of job.
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.