Good grades and standardized test scores are necessary, but they are not the only qualifications admissions committees consider. Admissions committees pay attention to extracurricular and community activities, research and work experience (paid or volunteer, relevant or not so relevant). Quality is far more important than quantity. Choose a few activities that really appeal to you and focus your spare time on them. Keep in mind that a good academic record is essential, and extra-curricular activities are not a substitute for good grades.
The Association of American Medical Colleges (AAMC) outlines core competencies reasonably expected of entering medical school students. Applicants who provide evidence demonstrating these competencies (key attributes and experiences) are most likely to have a successful application to medical school. These competencies are described in full detail here.
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The National Association of Advisers for the Health Professions (NAAHP) List of Summer Opportunities: http://www.naahp.org/Diversity/SummerOpportunities.aspx