My kids all looked at photo albums to get ideas. This worked so well with my first (who wrote about how playing with Legos prepared him for computer programming, and how missing pieces helped creativity) so we suggested it to the other two when their time came.
These essays do not have to be about important topics, or carry the whole weight of telling them who you are: the application has space for plenty of EC’s and so on, and there is also a supplementary essay option.
I tell students to write as if they were talking to someone in the room with them, and stay away from big words unless needed for meaning (E.B.White’s excellent advice in Strunk and White).
“Mundane” topics can make the best essays, they really can.