Another extremely popular service right now involves managing online ads for clients. If you have some experience with Facebook ad campaigns, 

You can manage ad campaigns for Google AdWords and Pinterest ads for clients. Other freelancers will work with ads on multiple platforms, while others specialize in one platform.

It is possible to learn this skill even if you don't have experience managing ads, and plenty of courses are available to learn it. With this service, you can earn a nice income (part-time or full-time) and learn a skill.