Learn things you think will help your career by creating useful projects for yourself or friends using technology you want to explore. Learning by doing is best but doing without your job's business requirements lets you actually explore.
If your job requires licenses to do it, if you can’t own them (afford them) yourself or the vendor doesn’t have a licensing model that allows you to get your hands on the equivalent technology for educational and non-commercial purposes, maybe that’s not a technology that you want to learn.
Be interested in technologies that will evolve and survive. We live in an era where often something great and simple becomes a piece of crap over time only to be forked and have the process start over again. There have also been cycles where the formalization and standardization of technology has led to complexity that makes it unattractive to new developers. Often they reject a standard and implement a new more straight forward approach to solve the immediate problem. The beauty and elegance of the solution for the initial use case draws contributors until the beauty is marred by attempts to re-add all the lesser use cases. There are so many ways for a technology to die or become dormant. Sometimes you can’t help yourself, but try to assess the future of a technology when creating synergy between your career and your side projects.
If you like the advice from Mr. Frank, you may like other business insights with these tags, and these special links:
Just-In-Time-Development
Risk-Voodoo-1
Risk-Voodoo-2