[krsna]

I absolutely agree with reading. But I'm surprised no-one's mentioned writing. I find writing (à la pen-and-paper) to be a great way to organize one's own thoughts and opinions. It's so easy to read someone else's opinion and assimilate it implicitly into one's own view. Writing, on the other hand, seems to allow for deeper inquiry into a subject.

[mikeleeorg]

Ah yes, great point. The critical thinking, introspection, and analysis that goes into writing definitely strengthens neural pathways.

You just reminded me of another one: teaching.

One study showed that older siblings, on average, tend to be slightly smarter than their younger siblings. One plausible rationale may be because they spend time teaching their younger brothers & sisters. http://www.bioedonline.org/news/news.cfm?art=3401

If you've ever taught someone a subject, and found yourself walking away knowing that subject better than you did before, you're experiencing a similar phenomenon.

The act of teaching a subject requires you to understand it well enough to be able to synthesize, then explain it to someone else.