These days, having good grades and being a hard worker is simply not enough to gain admission to medical school. Extracurriculars now play a huge role in your admission prospects, and having the right activities makes you an interesting, well-rounded applicant, hopefully giving you the edge needed to gain admission.
You have great grades and letters of recommendation, so who cares about extracurriculars? The admissions committee and your interviewers, that’s who! There are many students with good grades — the applicant pool must be narrowed down somehow. Having exciting extracurriculars that show off your leadership, compassion, motivation, sportsmanship, and personality can help them choose you over another student. As an added bonus, if you have an incredibly interesting extracurricular, it is likely your medical school interviewers will want to focus on it and hear more, leaving them less time to grill you on the merits of Obamacare!
There is no right answer to this question. If there was a list of all the "right" activities, the applicants would begin to resemble each other again, removing the utility of these activities to the admissions process. Instead, you want to select activities you are passionate about. You should also try to demonstrate dedication by having some activities that are lengthy, require commitment, and show leadership. Popular activities include scientific research, sports, students groups, and community service.
The one extracurricular activity that every student must have is some form of clinical exposure (with the notable exception of some MD-PhD applicants). Schools want to see that you know what you are getting yourself into and having these experiences will help you answer the age-old interview question: "Why medicine?"
If you are looking around for an extracurricular activity, remember it’s OK to choose something you enjoy. This will allow you to speak passionately about this subject. You'll be a great conversationalist in your interviews and your enthusiasm will shine in your essays.