I would strongly recommend learning new languages--both human languages and computer languages. The most beneficial ones will be the most different from what yo

I would strongly recommend learning new languages--both human languages and computer languages. The most beneficial ones will be the most different from what you already know, so if you know English and one or two European languages, try learning something like Chinese or Japanese. It's especially helpful if you can immerse yourself in the other language's environment for some period of time so that your brain is forced to start thinking in the other language. Language and culture shape how you think, and there is no substitute for learning a very different language and being immersed in a very different culture if you want to see how your thoughts and the thoughts of others are affected by those things. I think this exercise is one of the few in this thread that will actually challenge your brain sufficiently to make you significantly smarter as opposed to improving your mental toolset for thinking about the world. I'd also recommend reading up on behavioral economics, marketing, and design. This is more along the "toolset" line, but understanding why people make the decisions they do and how other people try to exploit those decision-making processes helps to understand a significant amount of otherwise relatively impenetrable behavior. Oh, and read How to Win Friends and Influence People if you haven't already. As someone else suggested, talking to people can yield a great deal of knowledge, but trying to think from the other person's perspective (a major theme in the book) can yield a great deal more understanding of people in general.