If you have your CNA/First Aid/BLS, some basic medical training, or EMT look for summer camp medical tent, or outdoor festival medical tent volunteer gigs. You can get a lot of hours in a short time.
Alternatively, if you worked as a live-in caregiver, you could get a lot of hours. Sometimes care givers take vacations on the summer so you could sub in for someone. You can advertise yourself on care.com, Craigslist, or call around to various home health, hospice, or case management agencies. Helping a sick, dying, or handicapped person in their home is a very intimate, often difficult experience and would not only be a lot of hours but a top quality experience.
Hospital volunteers often don't actually have much patient contact and doing a bunch of hours sitting around the hospital sounds like it could be super boring.
If you don't have your CNA or EMT, just spend that month taking a training course to get it. That will open up a lot of doors for you or future opportunities and really help you get your feet wet in the medical field to make sure you want to be in it.
Good luck!