Here are a few things that you can do :
C, C++, Java is a good foundation, next you need to understand how to make all the backends chat with each other, so you're looking for something like a Service Orientated Architectures. Best book imo about this subject is "Enterprise Integration Patterns" (it's a thick hardcover book). Then see what books you can find by Martin Fowler, he's written quite a bit on NoSQL and other Enterprise related topics. Cram some books on AMQP (RabbitMQ is a nice tool to have in your arsenal).
Get your hands dirty with Java8's Stream APIs and Lambdas, they're very useful. Learn about Spring, Spring MVC, Spring AMQP, Sprint Integration, then go and learn how to do the same thing using pure JavaEE just so you have a balanced view of both the Spring and JavaEE world. Work your way through at least another JVM based language like Scala.
Use Hibernate on MySQL, then swop out MySQL for PostgreSQL and benchmark each of them to see what kind of performance you can get out of your database layer via your ORM. Swop out Hibernate for pure JPA and see which one you like more.
Use JDO and build a Google App Engine application, then replace JDO for Objectify and see how much easier you can store / read / cache stuff on Google's datastore.
Jump between IDEs, Netbeans, Eclipse, Spring STS (Spring's zooped up eclipse), IntelliJ Ultimiate ... learn how to use VIM, then switch on the VIM plugin in each of these IDEs and use the VIM commands inside these IDEs.
Make sure you can work on other machines as well, Linux, BSD, OSX, Windows, etc. Learn to script on each of these OSes, write some bat files to do stuff for you in Windows, then bash files in Linux - Linux is really something you can't live without.
Get your hands dirty with some Python scripts which you import in Jenkins to do build automation for you. Write some basic PHP scripts to output stuff on a website, even though I'm way deep down in the architecture of stuff, I still have to tinker on PHP projects from time to time.
Once you have the backend mastered, start mastering DevOps, Docker is an awesome piece of tech you really need to master, make your Jenkins instance build docker images for you, push them to the Docker cloud and deploy them on both Amazon Web Services as well as Google Compute Engine.
Learn how to use JAXB, JAXWS or Spring Webservices to build a SOAP API, build JSON APIs in Spring MVC using Jackson, then swop out Jackson for GSON just for the fun of it and benchmark the two. Use SOAP UI's REST feature to load test your JSON APIs. Build projects using Maven, Ant and then build Gradle scripts for these projects to see the difference between Gradle, Maven and Ant.
Once you can build JSON APIs using Jackson, start with frontend technologies, get your hands dirty with GWT to see why you should avoid GWT for the better alternatives out there, then switch to Dart and start building some awesome stuff in Dart which you compile to JavaScript, get your hands dirty with AngularJS, by the time you get here, AngularDart should be ready as well. Rip out your Spring MVC + Jackson JSON API and redo it in NodeJS.
Understand how to do proper CSS3 and HTML5 and learn how JavaScript works at the core (which will help you understand jQuery better), then throw yourself in the deep end and try to understand the Shadow Dom which will help you understand how the whole Polymer with Polyfills fit together. Build some components in Polymer and re-use them.
Experiment with Bootstrap, Foundation and Semantic UI and see which one you like best. Try and make your Bootstrap / Foundation / Semantic UI app work in Cordova / PhoneGap and test it on your mobile phone.
I can probably go on, but these are some good pointers to get you going.