There's already a lot here and you probably stopped reading, but I couldn't resist writing a note to my 23 year old self:
- If you do anything, do this first one: learn how to work hard and stick with something. Learn how to turn off Facebook and control your addiction to social media. You'll instantly be ahead of a majority of your generation.
- Video games are a time sink. If you can play for an hour and say "That was fun, let's do something else" then you're ok. Otherwise, just get rid of them.
- Pick up an athletic hobby that you can do through the years, or your sedentary academic lifestyle will do horrible things to your posture, back, and gut.
- Learn how to deal with interpersonal conflicts. Read Crucial Conversations and practice - it's a lifechanger (shameless plug: Crucial Conversations Tools for Talking When Stakes Are High, Second Edition)
- Learn as much programming as you can. It's never a bad idea to learn a programming language (unless that language is COBOL—just kidding, you can even make money coding in COBOL). I've found Treehouse is worth the investment.
- Learn how to display data in R.
- Learn how to talk on the phone with people you don't know. It still terrifies most people. Get a phone sales job and quit after a few weeks.
- Start some kind of little side venture to learn entrepreneurial principles. Start a window washing business, buy and sell stuff on Craigslist, do anything. You might need them if you're in between jobs.
- Take a course on statistics and how they're misused, specifically how faulty causal arguments are made. It will change the way you approach almost any discussion, quantitative or not. (Read How to Lie with Statistics: Darrell Huff)
- Always have a private place where you can talk out loud to God every day. Even if you're not religious or don't believe in God. Talk out loud to the universe. Sort out your thoughts, verbalize them, and then write down what comes to mind. Something wiser than you is listening and responding, even if it's just another side of you.
- Use http://Mint.com to track your expenses. Make a budget and learn how to stick to it. Learn to live lean now while you can, so you won't be forced to later on.
- Learn a foreign language, but only if you anticipate using it. Sure, there are side benefits, but you'll forget it.
- Learn how to network. Read "Never Eat Alone." (plug: http://amzn.to/XQqfrM). Hint: it's about doing things for other people.
- Learn how to cook tasty, cheap food (If you buy one cookbook in your life, Cal Peternell: Twelve Recipes is a good one.) And make this bread. No really. http://www.steamykitchen.com/168-no-knead-bread-revisited.html
- Find music that inspires you and makes you feel alive. Listen to it every morning and if it's something embarrassing just don't tell your friends about it.
- Save up a few thousand dollars and go somewhere crazy with a few friends. Live in a hostel for a few weeks. Get malaria if you have to. It will change your life and you will think about it forever (not the malaria part - you'll get over that.)
- When someone promises you easy money, run the other way. Yes, it IS a scam.
- Read a book a week (I listen to Audible.com the car). When you get ridiculously wealthy, read a book a day. Read fiction too.