Would you like to enhance your threading, streaming (file, socket, etc), drivers, etc? Would you like to do embedded applications, desktop application, GUI or web pages?

When I want to learn a new language or experiment with different facets of a language, I find a problem to solve. I usually start off automating a manual process and build up from there, or rewrite one of your existing programs using a different design.

Learn another operating system. Each OS is different and will require different designs to accomplish the same thing.