I suppose it all boils down to logic. Logic is the underpinning of how we understand our world, and a few simple logical tools build most of the complexity in our day-to-day lives.
Language. Math, programming, and English or Chinese. Each of these three styles of language are for very different goals, but they function in similar ways. They are representational structures that allow named things to be organized and shared. I think a field of math, a field of spoken language, and a programming fluency will help people understand their world today. For example: Statistics, C++, and English. These three tools open up a lot of doors for the investment.
Critical thinking skills and emotional intelligence.