I'm in the camp that says the best thing you can do for a young kid is expose them to as many types of books as possible. If you can instill a love of learning/understanding/knowing in an eight year old, he or she will do your job for you.

Science also tells us that kids at that age need way more of a recess equivalent than they are allowed. Couple that with frequent school-related exposures to nature ... big field trips or even just little things (I remember my elementary school encouraging us all to go to a P-T meet and greet held not at my school but rather under the Congress Ave bridge to watch nightly bat movements).

So. Books, physical activity (the more student-organized the better ... by fifth grade we were setting up our own soccer matches during our 25 minutes recess period), nature.

mknod already hammered out math so I'll skip that.

Scientific method is vital. Teaching kids to apply it to not just science but things like conflict resolution, social interactions, decision-making... etc.

Finally, art.