If you want to get your ducks (or ECs) in row for BS/MD application, you need to be absolutely on top of this stuff. You need to know when the volunteer slots open and be first in line for them.
Also, you can do more than ONE activity during the summer. It’s usually not an either just X or just Y situation. I know D2 volunteered afternoons in Child’s Life and worked as a day camp counselor in the mornings and as an activities assistant at the local children’s science museum on weekends. She had lots of balls in the air–and managed not to drop any of them. It was great training for a future physician.
Pre-meds, med students and physicians all have to be pro at multi-tasking. (All healthcare providers do.)