There was a post on this subject but I can't seem to find it so let me repeat what I've heard...
Good electives outside of PM&R:
Neuro - learn good neuro exam, see acute strokes, TBI, etc.
Neurosurg - if interested - acute management of SCI, Spine procedures, etc.
Ortho - Both OR and outpt sports - good to learn about various procedures your rehab pts may have gone through, some physical dx skills.
Rheum - add to your diff dx list of msk pathologies
Cards/Pulm/ICU - learn vent management, medical emergencies, some cardiac rehab type issues.
Sports Medicine - Ortho or FP or PM&R - if interested
Pain Management - Anesthesia or PM&R - if interested
IM - was asked by one clueless interviewer what IM rotation I did my 4th yr.
Radiology?? - possibly to learn to read CXRs, Spine MRIs, Neuroradiological images, etc.
Obviously, each specialty will have its own idiosyncracies about how to approach a patient - also, vent management in SCI pts very different from standard ICU pts. BUT, good to be exposed. I guess just pick and choose... I've only done a few on this list.
Hope this helps.