I will echo what some others on this thread have stated, in that ECs are generally quite worthless. It may be tough to accept that after being on the premed treadmill, but it's true. Residency programs want people who are smart, hard working, and can get along with the rest of the team. Nobody gives a flip that you handed out teddy bears to volcano victims in Honduras.
You should still do some ECs while in med school, but not for box-checking purposes. Find a few things you enjoy and spend some time on them. Volunteer at some free clinics, play with children on the oncology ward, do some advocacy, whatever, it will help your sanity and your professional development more than it will help your ERAS application.
One thing I don't believe anyone has mentioned is the Dean's Letter. At the end of M3 your Dean of Student Affairs will write a rec letter for each student in the class. He or she will sit down with everyone's academic transcript, Step scores, and evaluations and bang something out. My Dean relied heavily on narrative evaluations from our clerkships, so screwing up on those could turn your letter into a pile of crap.
Short version: someone with influence is always watching.