Best answer by hamster
In order of importance:
- Algorithms (e.g. https://www.coursera.org/specializations/data-structures-algorithms)
- More Algorithms (e.g. https://www.coursera.org/specializations/algorithms)
- Unit Testing/Test Driven Development (if you can find anything)
- Source Control (e.g. Git) (if you can find anything).
- Interview/leet code practice (not a course recommendation, but definitely something you should do).
- And if you manage to do all of that: do even more algorithms!