Learning new languages
Just like it’s beneficial for the brain to learn several spoken languages, the same can be said about programming languages. The benefit might not seem big, but it adds up.
Each language has its own set of rules, of best practices, of approaches and paradigms. Learning more than one language will increase your ability to see a problem from different angles, to widen your perspective, to bring and apply principles from one language to another.
Adrian, in his post about being a developer over 40, gives advice as to learn a new programming every year and his approach is to build a simple calculator with whatever he learns — getting familiar with the IDE and APIs.