Well, my experience won't really help if you are looking just for something to do for the summer before school, but if you wanted to do something later then...
If you are thinking about volunteering abroad after or with school you could consider the Peace Corps (which I did and am doing again with grad school!!). Several schools of Public Health like Tulane, Emory, BU, UAB, Johns Hopkins, GW, SUNY, and a few others participate in a program called Masters International. The programs vary at each school, but, in general, you complete all or part of your coursework on campus and then serve in the Peace Corps which counts as a semester of classes, your hours of practicum, or some other trade off. Examples: At Tulane you do three semesters on campus and then join Peace Corps, you submit certain reports and your service counts as you last credits and you graduate while in the Peace Corps. At UPitt you go 2 semesters, serve in the Peace Corps, and then come back to campus to complete your last two semesters (so you can see how it varies).
Peace Corps is a 27 month commitment, but it's a super unique opportunity to learn a new language, integrate into a community, and make a sustainable difference to a group of people. In Peace Corps EVERYTHING is covered while you are in country (your airfare there and back, a stipend for living, medical expenses, etc) and you get a stipend of about $6,800 (after taxes) when you return (there was a recent "raise" for Peace Corps Volunteers, I received about $5,600 when I closed of service 2 months ago). You accrue $250 a month for it while you are serving, so if something happens and you can't complete your 27 months you still get the portion you have earned.
Also with Peace Corps your loans can be deferred, and if you have Perkins Loans they will forgive 15% of your debt for every full year of Peace Corps service you complete (so 30% for your 27 months - however, I think Perkins usually only lends about 1,500 or 2K to students, but still!). You don't have to do it with your school if you choose a place that doesn't do masters international, your loans can still be deferred and with all the health job sectors Peace Corps has it would be great experience.
Peace Corps application process is no joke though, it takes about 12 months (sometimes shorter, sometimes longer, but right now it's taking a while because of potential budget cuts). There's intensive medical reviews, etc, and the waiting can be frustrating, but in the end it is totally worth it (in my opinion). Plus, if you're in grad school you just start the application process about a year before you are available to leave and the wait will be easier since you have a reason to be in the states.
Anyhoo, Peace Corps isn't for everyone, but it's a good option for people willing and excited to make the commitment! I loved my first service and can't wait to do it again with Masters International at Emory!