How do I continuously improve my technical skills?

I've been working as a software engineer for around 5 years now. I'm on the verge of being a senior engineer.

However, I'm not sure how I can make sure I can improve my technical skills. Whether I'm working on personal or work-related projects, I feel like my skills are stagnant. They're not bad, but I don't feel a sense of improvement.

Should I read books? Go out of my way to do projects I'm not interested in / out of the tools I work on? Just learn new languages even if I don't need them?

Advice from more senior engineers would be very helpful.