We know that Christian McCaffrey can run the football -- he did 2,019 yards worth of that last year. We know he can catch it to the tune of at least 645 yards in one season. Returning isn't a problem for him, either -- see his 1,200 yards in that facet of the game.
We know that McCaffrey can dominate Stanford's offseason workout regimen in a way that no player before him has.
And we also know that McCaffrey can play the piano proficiently. Evidence can be found in this 2015 College Gameday feature.