My family has visited Chipotle on Halloween for the past 5 years as they offer great discounts and donate proceeds to worthwhile causes. The first couple of years the task was to dress up as a burrito for a free burrito. This meant putting foil on your head. The kids got a kick out of this.
In 2010, the promotion was to dress up as a processed food. We ended up dressing as 2 little chicken nuggets, fries and a shake. The intent was to highlight the horrors of processed foods.
In 2011, the promotion was to dress up as a farmer. This was pretty easy also and amounted to creative bandana tying.
The past couple of years have not had a theme, but Chipotle is still collecting proceeds in terms of $2 and this year $3 entrees for showing up dressed in costume.