Hi Jonathan!
That's a good one! I'll try to make it a proper list:
1) Learn the foundations of CSS & HTML without frameworks or pre-processors
2) Learn the foundations of vanilla JS
(Both of these will help you debug your code more easily and learn new frameworks/libraries/tools more easily.)
3) Learn some basic visual design and typography principles.
(Even if you never call yourself a designer these will come in handy.)
4) Practice writing and sharing what you learn.
(Communicating your ideas well helps others learn and is key to getting promoted.)
5) Read up on the history of the web/web design
(The web things that came before now are both interesting and informative.)
I could probably make a longer list, but I think that's a good set of 5 to have :)