Advice that my advisor gave me was to think about areas of medicine that could be pertinent to peds but not quite under the peds umbrella like derm and sports medicine. It's also a good time to strengthen some weak spots.
As a third year, I didn't have much elective time, but I did 2 weeks in peds endocrinology and I loved it. (Also from my AMA morning rounds email today-"Between 2001 and 2009, prevalence of type 1 disease rose 23% and prevalence of type 2 rose 21%, said Dana Dabelea, MD, PhD, of the University of Colorado in Denver, and colleagues during a press briefing at the American Diabetes Association meeting.")
I think Peds ER is a good rotation to have- it's a lot of bread and butter peds, a great place to practice clinical skills AND do some fun hands-on stuff- splinting, sutures (depending on the attending/family).
I did a PM&R rotation, which while I saw mostly adult patients, the attendings tailored their teachings to peds. It was good to force myself to rethink anatomy (muscle groups, dermatomes) and learn things like return to play rules.
-again- derm is good, infectious disease is generally high yield, tons of peds in ENT
-radiology, anesthesia- I think it is good to get practice in reading films and doing intubations/IVs.