There's two things I get flamed about, both online and IRL... And this is one of them. So I tread here knowing someone will be unhappy with what I'm about to write.
The MD/PhD committee at a large number of (most?) programs makes the decisions about who gets accepted to the MD/PhD program with little if any input from the medical school. Many of those committees don't really care about shadowing/volunteering/community service types of things, and instead are pretty much completely focused on the usual things--GPA/MCAT/research experience/academic potential.
So you absolutely can get admitted to top MSTPs with none of the volunteering/shadowing/other ECs. When I was a first year I went around asking some of my classmates about these sorts of things for SDN/my own curiosity, and several of them had none of these activities.
I still say to be on the safe side you should probably get 50+ hours of experience doing some kind of volunteering/shadowing. This should serve to satisfy the MD adcoms who could veto your application at some schools and who are aware that you are planning on a mostly research career and thus have extensive research experience. Though if your application really undergoes a thorough review by the medical school before going to the MD/PhD program (a small minority of programs, but everyone always throws UCSF in here), 100+ hours is probably better, and for the MD-only crowd I say that 500+ hours is what they should be shooting for to stand out.
In the end you can't be everything to everyone. Should I thus be recommending everyone have extensive shadowing/volunteering/ECs? I don't think so. I think 50+ or perhaps 100+ hours is a reasonable guideline. You can apply with none, and even in this thread we see people who were successful with none.
I also frequently hear that you should do volunteering/shadowing to know what you're getting yourself into as far as medicine goes. I personally think that almost no matter what you do as a pre-med, you have no idea what you're getting yourself into. You should probably have other ECs just so you aren't a very one-dimensional person. Then again, if you want to be a top physician-scientist researcher, it's probably better that you are very one-dimensional and don't like leaving the hospital or your lab
(half-serious on that one).
Ok... So for specific ideas as to what to do, I did Saturday night volunteering in the ED. I mean I had no social life in undergrad anyways, so that was about as cool as I was gonna get on a Saturday night. Nobody else wanted that shift, so I had free reign. I saw a lot of good stuff there. The only downside was my car breaking down in the summer in my college town and me wearing that ****ing candy striper outfit with no jacket trying to get my car towed/fixed in that outfit.
