If you're not needing to do a rotation in support of your app for a specialty, common wisdom is to sign up for something that is likely to help your future practice.
If you're going to be a generalist of some sort (FM, IM, peds, EM, heck even ob/gyn, etc) or a general cutter, some ideas include:
-EM (required at some schools, but I am of the opinion that the vast majority of docs need some exposure to and comfort dealing with medical emergencies. I saw someone complain of sudden onset of chest pain that had aortic distension and was at imminent risk of MI or dissection, while they were at the opthalmologist's office. They didn't seem to know what to do. Obviously that needed kicked to the ED, still. Emergencies are emergencies and can happen anywhere. I as a doc want some experience with initial care no matter what I'm doing.)
Most intern/prelim years includes an EM rotation - even psych. In fact, psychs are MD/DOs because they *do* need some basis in somatic medicine and recognizing medical emergencies.
-Rads
-I had to do a surgical elective, and did ENT. What I learned has frequently been useful in generalist practice.
-More ICU - again, how useful this is depends on how much your school exposes you to, and what you go into. It's useful for a lot of specialties. My school it wasn't a requirement, I didn't do it prior to my intern year, and I suffered greatly for it.
-Derm (again, more useful for generalists. Skin care is an entire major issue to be managed inpt, as a doc you will always be shown rashes, and in a lot of specialties it's more or less going to be useful. You aren't going to call derm for every rash, and you do need to know when to call them.
-I know psych students that did more neuro or a neurosurgery rotation. Again, might be useful for generalists too.
-These are just a few ideas. At my school there was the chance to take something 3rd year elective instead of vacation, and it freed up MS4 time. I tried to knock out a requirement and then used the MS4 time for extra subI, vacation, interviews. If you can do something you have to do anyway for graduation and save that elective time for later, that might not be a bad idea.
-However, you can also use this time to get exposure to something you are considering but wouldn't get a crack at until closer to ERAS. The more you can experience and learn before MS4 the better poised you will be for choosing/applying to a specialty.
