Oh MAN, I had a big long list here somewhere...

• Frequently expose myself to shocking/horrific pictures, so that I am generally less sensitive. I've been doing this for a while, watching horror movies while doing cardio exercise, and it's been going well. One might also try pulling pics from (WARNING) shock sites and using spaced repetition to schedule exposures.
• Become insensitive to exposure to cold water by, for example, frequently taking cold showers or ice baths. This apparently helps with weight-loss as well. I've done this with immense success. After you've practised this, you will literally feel like some weird heat is being generated from someplace inside you when are exposed to cold water, and not feel cold at all. See here.
• Become awesome at mental math. I've been practising squaring two-digit numbers mentally for some time (school, what can I say) and I'm really good at it.
• Learn mnemonics. I was fortunate to teach myself this early and it has been insanely useful. Practise by memorizing and rehearsing something, like the periodic table or the capitals of all nations or your multiplication tables up to 30x30 or whatever.
• Practise visualization, i.e. seeing things that aren't there. Apparently some people lack this ability, and I don't know how susceptible this is to training, so YMMV. Try inventing massive palaces mentally and walking through them mentally when bored. This can be used for memorization (method of loci).
• Research n-back and start doing it regularly.
• Learn to do lucid dreaming. Besides being awesome in and of itself, this can help you practise things or experience weird stuff.
• Learn symbolic shorthand. I recommend Gregg. I did this in my second year of high school, and it's damn useful for actually writing stuff and taking notes as well as as a conversation starter.
• Look at the structure of conlangs like Esperanto and Lojban and Ilaksh. I feel like this is mind-expanding, like I have a better sense of how language and communication and thought works after being exposed to this.
• Learn to stay absolutely still for extended periods of time; convince onlookers that you are dead. Being in school means you have ample opportunity for practice.
• Learn to teach yourself stuff. Almost everything you can learn at high school or university can be taught better by a good textbook than by a good teacher (IMO, of course). You can get any good textbook on the internet.
• Live out of your car for a while, or go homeless by choice.
• Can you learn to be pitch-perfect? Anyway, generally learn more about music.
• Exercise. Consider 'cheating' with creatine or something. Creatine is also good for mental function for vegetarians. If you want to jump over cars, try plyometrics.
• Eat healthily. This has become a habit for me. Forbid yourself from eating anything for which a more healthy alternative exists (eg., no more white rice (wild rice is better), no more white bread, no more soda, etc.). Look into alternative diets; learn to fast.
• Self-discipline in general. Apparently this is practisable. Eliminate comforting lies like that giving in just this once will make it easier to carry on working. Tell yourself that you never 'deserve' a long-term-destructive reward for doing what you must, that doing what you must is just business as usual. Realize that the part of your brain that wants you to fall to temptation can't think long-term - so use the disciplined part of your brain to keep a temporal distance between yourself and short-term-gain-long-term-loss things. In other words, set stuff up so you're not easy prey to hyperbolic discounting.
• Learn not just to cope socially, but to be the life of the party. Maybe learn the PUA stuff.

• That said, learn to not care what other people think when it's not for your long-term benefit. Much of social interaction is mental masturbation, it feels nice and conforming so you do it. From HP and the MOR:

  For now I'll just note that it's dangerous to worry about what other people think on instinct, because you actually care, not as a matter of cold-blooded calculation. Remember, I was beaten and bullied by older Slytherins for fifteen minutes, and afterward I stood up and graciously forgave them. Just like the good and virtuous Boy-Who-Lived ought to do. But my cold-blooded calculations, Draco, tell me that I have no use for the dumbest idiots in Slytherin, since I don't own a pet snake. So I have no reason to care what they think about how I conduct my duel with Hermione Granger.

• Learn to pick locks. If you want to seem awesome, bring padlocks with you and practise this in public :P

• Learn how to walk without making a sound.
• Learn to control your voice. Learn to project like an actress. PUAs have also written on this.
• Do you know what a wombat looks like, or where your pancreas is? Learn basic biology, chemistry, physics, programming, etc.. There's so much low-hanging fruit.
• Learn to count cards, like for blackjack. Because what-would-James-Bond-do, that's why! (Actually, in the books Bond is stupidly superstitious about, for example, roulette rolls.)
• Learn to play lots of games (well?). There are lots of interesting things out there, including modern inventions like Y and Hive that you can play online.
• Learn magic. There are lots of books about this.
• Learn to write well, as someone else here said.
• Get interesting quotes, pictures etc. and expose yourself to them with spaced repetition. After a while, will you start to see the patterns, to become more 'used to reality'?
• Learn to type faster. Try alternate keyboard layouts, like Dvorak.
• Try to make your senses funky. Wear a blindfold for a week straight, or wear goggles that turn everything a shade of red or turn everything upside-down or an eye patch that takes away your depth-sense. Do this for six months, or however long it takes to get used to them. Then, of course, take them off. The when you're used to not having your goggles on, put them on again. You can also do this on a smaller scale, by flipping your screen orientation or putting your mouse on the other side or whatnot.
• Become ambidextrous. Commit to tying your dominant hand to your back for a week.
• Humans have magnetite deposits in the ethmoid bone of their noses. Other animals use this for sensing direction; can humans learn it?
• Some blind people have learned to echolocate. Seriously.
• Learn how to tie various knots. This is useless but awesome.
• Wear one of those belts that tells you which way north is. Keep it on until you are homing pigeon.
• Learn self-defence.
• Learn wilderness survival. Plently of books on the net about this.
• Learn first aid. This is one of those things that's best not self-taught from a textbook.
• Learn more computer stuff. Learn to program, then learn more programming languages and how to use e.g. the Linux coreutils. Use dwm. Learn to hack. Learn some weird programming languages. If you're actually using programming in your job, though, make sure you're scarily awesome at at least one language.
• Learn basic physical feats like handstands, somersaults, etc..
• Polyphasic sleep?
• Use all the dead time you have lying around. Constantly do mental math in your head, or flex all your muscles all the time, or whatever. All that limits you is your own weakness of will.

So anyway, that's my idea-dump. Tsuyoku naritai.