Best answer by Gordon
Software development is a broad combination of skills, which makes it hard to give general advice.
You need to answer some questions for yourself: what different would you like to be able to do? How will you learn? You could spend all of your time studying, and not really be ready for anything specific.
Here are a few possible suggestions.
Learn a language that is very different from those you currently use: I often suggest Haskell as a language that'll make you think differently.
Pick a skill area such as data visualization and use two different libraries to develop basic competence.
Think "If I were looking for a new job, what would I like to be able to show", develop those skills and a portfolio that a potential colleague can study and get it on github. Put a pointer in your resume.
You'll keep learning in a software career. Devote 10% of your time to getting ready for new things. And have fun with them.