Practice unusual events during class instruction time. Designate a time-out place for misconduct during the field day. Explain its purpose and the rules to all students who will attend. Use an appropriate name to support the theme, e.g., Bull Pen. Have this place monitored by a teacher, not a parent.
Bag lunches work great.
Consider doing a cookout as a finale. Plan for music and a speaker system. Have handouts ready for teachers. Include schedule of events, a map, who starts where, opening and closing procedures, and any other details. Go over the handouts with entire faculty.