Many state and local health departments offer summer internships, and some of them are paid! I did an internship (which was paid) with a major city health department during my undergrad, and I do believe it helped in my MPH application process as well as my understanding of the field. You can google the names of your state or local health departments with the word "internships" or look on their websites and they'll often have information posted or somebody you can email to get information about possible positions.