Since smartness happens in the brain mostly, specifically neocortex, brain, memory, analitical thinking concepts are good places to start, So that you can understand why most of the advices are good. In order to give you a list.

Social skills:

- Learn to really listen other people. - Learn to ask correct questions - Body language, gestures - Meeting new people from different cultures, especially people don't believe in prophets and hell and heaven stories

Improving brain:

- Playing a musical instrument - Learning another spoken language - No TV, no football, soccer bullshit, not too much porn, talking about girls & boys or cars& houses, celebrity etc.

Gaining Knowledge: - Mathematics, Physics, Programming, Astronomy, Biology, Paleontology, Evolution, any science branch you are interested.

- On the other hand, i personally think, the best way to learn about a different culture is to travel whether it is local or not.

Some good resources:

- Here of course - wikipedia - documentaries - coursera - youtube, if you can avoid stupid girl podcasts, otw you will end up with bad recommendations. - Follow inspiring people on twitter

I will stop know, i also want to learn what other sugesstions are :)