If I were to redo my undergrad I would have exclusively volunteered at our local free health clinic. The differences between hospital/clinic volunteering is vast. You get tons of one-on-one patient interactions doing free clinic work. At the hospital you are more likely to stock supplies, do clerical work, and just observe others. I always felt I was in the way, although I understand many have had great experiences (I feel this is only after long-term commitment though). At the clinic you are an integral aspect to their operation, for they rely on volunteers like you/me in order to reduce costs. You'll feel needed, you'll be busy, and you'll get a lot out of the experience.
The previous posts are spot-on. You'll get to take histories, basic vitals (maybe even a full physical if you're experienced enough), review charts, and relay the case information to the attending physician. In my experience, the pre-meds were treated as how I would expect a medical student to be. We were actually utilized.
On a side note it would definitely be beneficial to have a grasp on Spanish (other languages are often needed as well too). The communication barrier can be a problem as there will not always be proficient translators available.
