As someone in PM&R, I'll tell you it depends on what you're interested in, the rotation setting, and each particular preceptor.
Outpatient PM&R: more general musculoskeletal medicine, potentially lots of back and neck pain, EMG/NCS, sports, peripheral joint injections, possible pain/spinal injections. Depending on the outpatient PM&R clinic, might see some neurorehab with TBI/SCI patients, botulinum toxin injections, baclofen pump refills.
Inpatient PM&R: Likely see lot's of strokes, post-ortho patients, maybe some traumatic brain injury, spinal cord injury, neuromuscular disease rehab (AIDP, CIDP, Myasthenia Gravis, etc.), maybe debility.
If these sound interesting or beneficial at all, I'd recommend doing a PM&R rotation.