I've always thought that if you have average grades, average board scores, average letters and no publications, but interview well, be able to carry a conversation, be well liked, you can land a spot. Obviously, if you are the best in everything, your position is better, but c'mon.
I don't think there are particular "activities" to be competitive. In ortho, ER, surgery, particular IM or Ped subspecialties, you need to do certain things but not FP (in my opinion). The field is too diverse and the people are too diverse/versatile to generalize.
I think FP's value being "well-rounded" in general. Yea why not, some research, some community service, some leadership, some traveling/foreign languages, some public health projects, some primary care stuff, some non-FP activities. I don't know. I think FPs focus on things you have accomplished versus things you haven't done.