Ask HN: What are some valuable projects or courses to become a back-end dev?

As a software engineer with several years of experience in mostly front-end & mobile, with some back-end work through full-stack roles, what are some great ways I can show off back-end skills on my resume? I'd like to transition from front-end to back-end with my next role.

I'm already brushing up on system design problems, but it would be great if there were some project ideas, courses, or certifications that the HN community finds very valuable.