Millennials enter the workforce with limitless dreams but limited options for high-paying jobs. Their median household income is about 20 percent less than Baby Boomers earned at a similar stage in their lives, and many Millennials wrestle with staggering student loan debt and skyrocketing healthcare costs.

As dispiriting as this situation might sound, Millennials can kick open the doors of professional opportunity thanks to fast-moving technology advancements that have created countless side hustles. About one-third of workers have a side hustle, according to a CareerBuilder survey of 3,200 private sector workers. Millennials are particularly active in the growing trend, with 44 percent of 25- to 34-year olds and 39 percent of 18- to 24-ye...