You need:
1. extensive clinical work (which can include research and/or volunteer work)
2. extensive research work (which can include clinical experience)
3. extensive volunteer work (which can include clinical work)
I believe that a strong applicant has significant experience in these three areas, and, as you can see, some of them can overlap. Overlapping is ideal.
Spreading yourself too thin can look like lack of commitment and application padding. I think you can do all three of these things with a serious, long term commitment to TWO activities -- just make sure the two things check all three boxes.
Personally, I think pure shadowing is kinda BS. There are many, many ways to "observe" what doctors do while actually DOING something yourself.
I did two years of research (with poster and publication) that included, as part of the job, daily patient contact and testing. (check: research and clinical)
I spent 6 months in a South Asian country living with a family and working in a rural hospital. My primary duty was teaching English to doctors and nurses in the evening. My days were spent in the OR and elsewhere right next to the surgeon translating everything he was doing into English. (check: volunteer and clinical "shadowing" and getting out of your "comfort zone")
My MCAT was good and my GPA was "marginal." I got 10 interviews and 6 acceptances. Every interviewer talked constantly about my experiences. Never mentioned my grades.