If you play blackjack for money, you can improve your EV significantly by devoting an hour to learning (most of) basic strategy. If you already know basic strategy, you can learn the basics of card counting in an hour. (Actual card counting requires significant practice after you "know" it in order to maintain a correct count under stressful conditions.)
Agreed on learning blackjack. I went to Las Vegas about a month ago and spent the four-hour plane ride trying to memorize some of the pattern in a blackjack strategy chart. I didn't memorize all of the actions for every possible hand combination, but I learned enough to be dangerous (read: enough not to make dumb/obvious mistakes). I ended up quadrupling my money (only a few hundred dollars) and have been playing really well on a blackjack app on my phone.
Kelly criterion - what can you afford to invest.
Polya's problem solving method.
Salary negotiation skills.
Rich Hickey's Hammock driven development (for non programmers too).
No Limit Holdem Poker flop and turn odds calculation.