In terms of a foundation, at an early age, I'd say emotional and moral development is most vital. My kids are damn smart and I didn't have to do much about it. However, I had to spend a lot of effort teaching them to not be envious, not give up when things are tough, not to take insults too personally, to get boring stuff done first (like showering), and so on.

If the stuff you have planned for them may not be fun, that makes emotional development all the more important.