Internships are a great way to determine what you want to do once you graduate and bolster your application for grad school. Opportunities vary based on profession but you can do anything from volunteer in a hospital (if you are interested in medicine and health) to work for a newspaper or news station (if you are interested in communications, journalism, etc. ) While interning, you want to remember to work hard and begin networking. These employers might eventually write you a letter of recommendation, become a mentor, or hire you for a full time job!
A graduate admissions counselor will look at your GPA and courses - and test scores - if applicable - when considering you for admission. If an essay is required, they will also read that to determine why you want to be part of a program and how you will benefit from it. Internships, clubs, leadership roles, etc. will only strengthen your candidacy.