You don't have to do anything, but if you do I would consider doing a structured summer program. It can be in FM or it could be in anything else you find interesting.
I did one in a topic I really enjoyed (psychiatry/public health), but way outside the field I am going to apply for, and it was really fun! Feels like it really broadened my horizons. I got to go to interesting didactics by really cool/important people, shadow in some cool things (also helped me confirm what I wanted/didn't want to do), got paid a few thousand bucks, and got a shiny little abstract to my name. Didn't even take up my whole summer.
You only get one shot at doing so much weird stuff in your life. Take advantage.