Well since you are applying next year, I'd wait to see if there will be optional essays for those schools. Harvard had no optional essay for example, but there is a small text box for you to add additional info.

Anyhow, overall, there are basically two types of Optional essay:
1. Dealing with issues - These optional essays have questions worded in such a way that it's clear that you should only write the essay if you have ISSUES, meaning medical problems, want to explain poor grades, why you don't have a recommendation from your current employer, why you have been unemployed for the last year etc.
2. Open Ended - This type of question basically asks "Do you have anything else you want to tell us?" And here you are free to choose the topic at your leisure. And in this case if you have nothing to "explain away", try and choose a NEW and unseen aspect of your profile to write here.