Hi Songbird,
These essays are a pain the You-Know-What because they are so short. I would advise to mention the presentation that gave you a relevant epiphany. It has to be summarized in a sentence or two, it's tricky. If none of them gave you a jolt, then mention how the content or themes are connected to your future research goals. If they are just cool, in general, then leave them out. Instead, use the space to discuss your more recent work or activities pushing towards a PhD.
Good Luck,
DL