I can't resist:
-renaming cities I conquer to fit in with my Civ's naming scheme
-unleashing my Abrams and Bradleys on spearmen, macemen, etc.and enjoying myself
-not allowing the spread of religion beyond those controlled by my country
-planning conquests in order to make my country better shaped rather than by a grand strategy
-not sharing techs for free (even with my "friends"), allowing open borders, or selling techs for anything less than an exorbitant amount of gold
-sending my privateers to pillage fishing boats even though I don't need the gold and barely know, let alone am hostile towards, the Civ I'm ruining with my pillaging