Watch video's on YouTube or TED: Ideas worth spreading, youtube because sometimes it is more fun to watch things about a subject you love than to read about it. Second, ted because you can learn alot from people's experience.

Read the news and if u can in a different language.

Do things ! You'll learn alot from your mistakes and your experience. By trying things you wouldn't do on your own. I've found each time you learn things from your own experience you will find yourself more complete and satisfied !

Edit
The best way you can improve is by yourself, you can learn a lot from people but, it is so important that you realise that u have learned, that you have improved. Keeping your mind clear, stand still by the moment and actually think about. It probably is the best reason to improve and to actual get smarter. In life we have time enough, analyse everything you encounter. It will improve the way you think, rational. It will also improve your actual skill. Be eager to learn, be the first to critique yourself and be the first to correct yourself.
There is a difference between knowing something and understanding something!