-Neuroradiology
-Neurophthamology
-Neurosurgery
-Neuropathology
-ENT (Neurotology)
-Cardiology (lots of cross-over with Stroke medicine)
I think the most important one to take would definitely be the Radiology elective. If you've got extra time, then I think every Neurology resident could benefit from an Ophtho or NSG rotation, too.