Hey there,
If your GPA is a weak point, I would consider taking a core course such as Epidemiology. Regardless of your intended concentration, you should make sure that there is a quantitative component to the course, since your GPA is what's lacking. Your other extracurriculars will speak to your interest in your desired field.
Most public health graduate programs will allow you to take a course as a non-degree seeking student. This will also allow you to receive a grade, and possibly apply it towards grad studies. Taking a 'non-credit' course won't help much if you don't receive a grade.
Again, I would suggest that you take a graduate level course, but there may be some merit to taking an upper-level undergrad science course. Or, there may be some graduate-level diploma programs that you can take courses from. I know Hopkins has some distance courses, and these would look great on an application.
What range is your GPA? Is it below 2.5? Below 3.0?
Most PH schools highly value work experience, so make sure you beef this up over the next year, and really emphasize this in your statement of purpose.
Oh, and don't forget to focus on the GRE. Scoring well may compensate for your GPA, and will at least give you something to argue with (a Stats course isn't a bad idea either).
Good luck!