It really depends on what your home program is like. If you have a strong functional neurosurgery group, I'd recommend spending time with them, for your purposes. DBS is being utilized for a variety of refractory psychiatric conditions, and the functional guys tend to love teaching neuroanatomy. Also spend some time with the pediatrics peeps, at least at my hospital they tended to place shunts for NPH, it's really interesting to see (for the first few times). Finally, you can never go wrong with tumor/vascular, it always requires a conversation about anatomy. Nothing wrong with spine, it might also help you as you'll see chronic pain patients in psychiatry.
For my 4 week rotation, arrive at 5 every day, round on ~3 patients, present at 6 when running the list, finish by 7. Head to the ORs until the room finishes its last surgery. Check in with the residents. Get out by 6-7 usually. We were also there for q3 overnight call. I saw a lot of tumor, spine and trauma, some vascular and an occasional DBS.
It just depends on what your neurosurgery department is heavy in, and also, if they'll let you customize your OR schedule.