Don Quixote: The Don almost always represents the idealistic.  His Lady Dulcinea, who he imagines to be a noble princess, his nag who he believes to be a noble steed, and the other figments of his imagination (windmills, etc.) further the ongoing metaphor for his madness.

Sancho: Quixote's squire, on the other hand, usually symbolizes realism (though most critics overemphasize this point, forgetting the countless times that Sancho himself is convinced of his master's words).  This is most clearly shown when Sancho tells his master, "I sometimes think that all you tell me of knighthood, kingdoms, empires and islands is all windy blather and lies" (Book 1, Part 15).

Acorns: When Don Quixote sees acorns one night in front of an open fire, he is reminded of the Golden Age of men, where corruption didn't yet exist.  The acorns serve as a metaphor for such an era.