Upvoted for an excellent expansion on the OP's comment. I will second world travel; I backpacked through India for two months on my own (made a lot of friends though) on a budget. I've also been sent out on many survival trips, while not world travel, you see a side of life most urban dwellers never even scratch at.

I will strongly second the "no drugs" advice. I experimented and quickly stopped because I noticed - more than inhibiting my mental capacity - it ruins your life and makes you dependent (rather than autonomous). Autonomy is the watchword of an adept mind!

Read. Read. Read. But, don't believe everything you read! Part of being intelligent is knowing when to accept a piece of thinking and when to question it; when you do question it, dig into it deeply until you come out the other end with a whole new set of original conclusions that were inspired by one dubious conclusion.

I will strongly second the advice to foster many interests. You interact with people that have points of view (goes with world traveling well) you would never consider. Also, don't forget, "weirdos" or "eccentrics" have much to offer - just be sure to follow your gut when interacting with them (some have misguided intentions, others are pure as gold).