When doing volunteer work, I'd recommend doing something unique. Probably 90% of medical school applicants have volunteered in the ER or have pushed patients around, so don't expect anyone to be impressed by these activities. Here's what I did:
I volunteered in the geriatric department of a local hospital. After spending a couple of days learning about the setup of the facility and observing therapy sessions, I noticed that recreational therapy could be done by individuals with no real formal training. So I spent a couple of days observing the recreational therapists at work in hopes of coming up with my own idea for a new therapy. After about a day or two, the idea struck me. I decided I would design a recreational therapy based on old-time radio programs. My plan was to purchase a few old-time radio programs (Jack Benny, Bob Hope, etc.), play them for the patients, and then discuss the programs with the patients. I would ask them about what was going on in their lives when this program originally aired, etc. This type of therapy is called reminiscent therapy and is a genuine type of therapy used by therapists world over. It stimulates the minds of those going through rehabilitation and is considered very therapeutic.
So I typed up this proposal (goals, etc.), presented it to the head of the recreational therapy department, and it was approved. I started my therapy sessions and it was a big hit. I would often have five or six patients listening to these programs using headphones. Then I would just spend the afternoon visiting the different patients and talking about the programs. They loved it.
The local newspaper caught wind of this and they came and followed me around while taking pictures and interviewing me and the patients.
Anyway, when I had my medical school interviews and was asked about my volunteer experience, the committees were always impressed by this activity. It was a helluva lot more fun, more educational, and more meaningful than pushing patients around the hospital in wheelchairs.
In sum... do something original and meaningful, guys. When you apply, you want to stand out. If everybody else is doing the same thing you are doing, then it is time for you to find something else to do.