How can back end developers 'practice'?

It seems everyone says to build something if you want to learn a technology. This is an issue for me because I work on server side code, mainly working with databases and how to optimize their performance. I am not a UI/UX person at all. So to build a website or project just feels... disjointed for me. I care a lot about the whole product, but I don't want to do front end work. So what sorts of things should I be considering as side-projects to boost my skillset?