I'm a Software Engineer who works mostly on Java. What can I start doing now, that will help me in my career in long run?

If you use windows, start coding with C# in visual studio.

Its almost identical to java and the switch over would take hours if not less. You can download the Free Visual Studio Community 2015 and be up and running quickly.

With that, you can try building MVC web apps and desktop applications that a lot of companies hire for.

Don't forget to read about and use NuGet to pull in packages to easily pull in tons of free addins for your applications.