If you are considering doing an IM residency (which it sounds like you are based upon your interest in GI and cards), a VERY worthwhile elective would be to do a month of Critical Care (e.g. in the ICU/MICU/whatever). Do so in a "closed ICU" that has its own unit team versus an "open" ICU, where anyone can admit. It is literally a living physiology laboratory. It really will help you take care of the far end of the disease acuity spectrum. There is also (depending on where you train) a tremendous opportunity for procedures (e.g. chest tubes, central lines, A-lines, possibly doing intubations, and of course, ACLS protocols).
I think that one of the best electives to prepare you for an IM resiency is ID, so that you won't be consulting on every one of your cellulitis patients (as some attendings seem to like to do ). It's also very useful for step II studying purposes too. Radiology is another nice fourth year elective for people going into IM. Finally, I'd try to do a sub-i in the MICU/CCU too, so you will be prepared in case they throw you into the ICU in July when you start your internship year.
Would anyone recommend doing two IM sub internships the fourth year?
Also, I hear that doing CCU immediately in the beginning of your fourth year is risky second to not having enough experience to handle some of the cases. Would it be wise to finish your Sub Internships first, possibly ER, cards, or anesthesia, before CCU?
Do your sub-i first or second. If you're not ready after 3rd year, you're not going to be any better of in October. Remember, it's a "sub-I" but they will not forget that you are still a student. Hard work is more important than pure skills.
ER, Rads, Anesthesia, ICU are all valuable. Rads especially has been helpful to me. So often we forget that x-rays are not "magic" and reading films is a good way to learn what they can and cannot do. Derm and Optho are nice if you can fit them.
Try to do some outpatient medicine rotations to make sure you like clinic. Personally, I hate clinic, so I guess at least I know that I'm going to hate it during my residency Prolly more to due with the fact that they are not "my" patients during med school.