OP my vote is research. Ideally clinical retrospective research that leads to a publication. Email your Peds department and see if anything is available. I didn't get it done early and now I'm scrambling to pull something together before the match.
Also don't let people tell you not to worry about match because you want Peds. While Is is easy to match into a Peds residency, it's not easy to match into a GOOD residency.