Minerva was of course there to chaperon but in the spirit of Halloween (and to show she's not a complete disciplinarian), she dressed up as a quidditch player; complete with a set of quidditch robes, broom, and a quaffle tucked under her arm. At one point she charms the quaffle to disperse candy to all the students. Though she also reminds students of the rules as she mingles in the crowd trying to keep the rule breaking to a minimal, she lets tiny infractions go though.