It is not so much a 'side hustle,' as it will take thousands of hours to become proficient, but I trade stocks on the side of my day job. I'm an introvert so reading and doing research in my free time, rather than being around a bunch of people, fits my personality. The actual execution of my strategies only take a few minutes each day, but I spend most of my free time (which isn't much these days with 2 small kids) reading books and testing out new systems.