do something you are excited about. if you want to get in the hospital and be a CNA, then by all means do it. but, i would stay away from choosing something just because it will look good. i haven't done the residency interview thing yet, but i have had lots of high-level job interviews, and that is pretty much what a residency interview will be. having something on your residency application like that you actually doesn't do much to set you apart, which is what you want. lots of med students have the obligatory "clincal exposure" before and during med school. so, do something you can get excited about, and something different than everybody else, if you can swing it..
i personally would never do the nurse aide thing, mainly because the job sucks, in my opinion. hard work, low pay, and you really don't get much medical experience. just sponge baths, making beds, emptying bedpans. i hold the people who do that in the highest esteem, especially because i just could never do it. they are stronget people than me.
i guess what i am trying to say is, do it if you want to, and you can get excited about it and communicate why it was exciting to you to your potential employer. otherwise, do something else that you can show interest in your field in and show genuine excitement. shadow a doctor, volunteer at the STD clinic, be a lifegaurd, get you EMT certificate, work at a soup kitchen, be a lifegaurd...the list is endless. i personally wait tables for money, and then do some volunteer stuff that i am interested in, like shadow surgeons. but, i also have about 8 years of EMT experience, so i don't really need the clinical exposure.
sorry for the long post, hope it helps...