If you want personal development, the best way to do that is to seek out the things that you believe you are no good at, and do them. For example, if you've always identified as a shy, introverted kid, the best thing you can do is to talk to strangers, make some friends, organize some events, and generally become the life of the party. If you've always identified as a learner and not a doer, the best thing you can do is take on a task that seems both pointless and challenging, do it to the best of your ability, and see what you learn from it. The way you phrase your question makes me think that you identify success with
knowing things, rather than
experiencing things or
doing things. So if you want to grow as a person, the most productive things you could be doing are to get way out of that comfort zone and see what happens if, say, you traveled to another country where you don't know anyone, or if you took on a highly-visible project where you might fail.
If you want to seem smart at cocktail parties, the best way to do that is to know just slightly more than your friends at all the things they care about. Some resources for that: Wikipedia, pop science magazines like Discover or Popular Science, Mythbusters, Science Reddit, and all the stuff linked from there. Also stay up to date on your celebrity gossip, pop culture, and sports scores.
If you want to make money, the way to do that is to pick an area that is currently in demand in the economy, specialize in that, and go deep, enough that you have more knowledge than anyone else. Computers are hot right now, as is SEO and online marketing. Biotech may be hot in the near future, as is clean tech. You basically want to deepen your knowledge as opposed to broadening it. Whenever there's something you don't understand - and I mean don't fully understand, it's easy to have a superficial understanding of things - go seek it out and learn as much as possible about it.
Learning about something for achieving an objective is similar, but the objective is chosen by you instead of by the economy.