As a nontrad student, you have had a lot more time on your hands to investigate medicine and ensure that you really want to have a medical career. Not having clinical experience is a red flag that you might not understand what you're getting into.
Personally, I believe that volunteering is critical if you are interested in clinical medicine. If you want to be a MSTP or other researchy doc, then you need serious research chops instead.
I worked FT, took bio lec/lab, chem lec/lab, and volunteered at three different hospitals on varying schedules, all at the same time. You don't get any extra points for overloading, most people just spend their time on one volunteer experience.
I picked one volunteer post in primary care, one in hospital care, and one in a hospital pathology lab. I wanted to cover all the bases. I loved all of these little jobs and still keep up with them when time permits.
I have been told by a med school interviewer that primary care, particularly with the underserved, is a very big plus. This interviewer was concerned that many students hadn't worked with very sick, very poor people before.
If you do a volunteer experience, try to get a setup where you can get a great recommendation out of it. If you're just showing up on an ER floor with a rotating series of nurses, there's no one person that can write about what a great doctor you will be. Try to have at least one volunteer posting where you get a decent amount of face time with one physician who can then write an engaging and personal letter about your exploits.
Your top priority should be your enjoyment, not a recommendation or a line in the "extra activities" section of the AMCAS form. If you don't like the volunteer post, ditch it and find something new. You may even find that medicine is not what you expected.
Uncomfortable hospital experiences help you confront your doubt about becoming a physician (and you will feel this!) which will make you a better interview candidate when they ask, "why medicine?"
