Maryann Aita, Writer, editor, and 4-year speech and debate competitor
This is a great question because I think it's always important for high school students to learn about the types of skills that a career requires. In general, it's best to have variety of extracurricular activities to show colleges that your daughter is well-rounded and to expose her to a range of skills and interests. For adverstising, specifically, both theater and speech and debate would be great extracurriculars to try, as others have said, but I'd also suggest some activities that involve publications.
A school newspaper or yearbook committee can be a great way to experience working under deadlines, making pitches, and working with superiors (editors) to make a final product. She could also get the other side of the advertising world by seeing what kind of ads get placed in the paper. If she enjoyed this and stuck with it, she could work her way up to being an editor so she can get a feel for managing people and working with her team to make something coherent. Often, students on yearbook or the newspaper will communicate with the student body or conduct interviews, which are also great ways to practice public speaking skills and to develop professional writing skills (valuable in any profession).
Another option, depending on how outgoing your daughter is, would be to contact an advertising or marketing firm where you live and see if they'd let your daughter shadow an employee or come in to speak with someone about what she can expect. I made my own internship at my County Attorney's office in high school and it really helped me learn what my own strengths were and to get a feel for the professional world.
Although not an extracurricular activity, I'd encourage your daughter to take any business, marketing, or psychology classes she can as these are all useful in advertising. Even a class in photography and visual art could be beneficial when considering how information is presented.
Because advertising requires a broad set of skills, many activities will be useful, but I hope the ideas listed here will get her on the right track.