i went with things that were about me, but not necessarily medically-related.
example: i like bowling, so i considered writing about it, how it related to who i was, etc. etc. i didn't go through with this, but hopefully you get the picture.
i was given the advice that you can write about what you are doing this summer, and what you've learned that's related to medicine. i just started my summer activities when i submitted my primary, so i thought i'd give them an update.
Write anything that you haven't mentioned in your AMCAS that would help with your candidacy such as why you would be a good fit, what have you done since you submitted AMCAS, and so forth