My terrible advice: whatever you enjoy most. You're going to enjoy the course, you'll retain the most, and hopefully you put to use the most in your new job.
You can never go wrong with data. So here's a few ideas:
- Google Analytics academy: https://analytics.google.com/analytics/academy/
- Coursera has several good courses: https://www.coursera.org/courses?query=business%20metrics
- Udemy as well: https://www.udemy.com/course/marketinganalytics/
A few random online classes that I've recently finished. And, I prefer Coursera than actual certificates, but that's just my personal opinion.
- Agile meets design thinking: https://www.coursera.org/learn/uva-darden-getting-started-agile?#enroll
- Aligning business, brand and behavior: https://www.coursera.org/learn/brand
- Viral Marketing and How to Craft Contagious Content: https://www.coursera.org/learn/wharton-contagious-viral-marketing
Some other ideas:
- Hubspot Academy: https://academy.hubspot.com/certification-overview
- Marketo Certification: https://www.marketo.com/education/marketo-certification/
- Facebook Marketing: https://www.facebook.com/business/learn/categories/introduction
- Twitter flight school: https://www.twitterflightschool.com/student/catalog