As you know, Harvard has only one optional essay question at the moment:
"You’re applying to Harvard Business School. We can see your resume, school transcripts, extra-curricular activities, awards, post-MBA career goals, test scores and what your recommenders have to say about you. What else would you like us to know as we consider your candidacy?"
Currently, I am thinking about answering it by detailing selected elements of my written application that I feel could not be presented in necessary detail due to character length restrictions, e.g. concrete responsibilities and outcomes of an extracurricular.