Definitely agree with the comments above of not doing too many extracurriculars. A lot of SDN'ers seem to be the overinvolved overstressed type that do everything, so watch out for that 🙂
I would think that the purpose of extracurriculars included in the application is to make sure you are a well-rounded person who enjoys more than just studying and shadowing dentists. Do stuff that you like, and try new things. If you can, try to keep your involvement going in the organization for as long as you can- you get a lot more out of it, and it shows your commitment.
I did shadowing, but also was involved with my fraternity, student government, residence hall government, new student orientation, orchestra, biology club and teaching assistant, philanthrophies like Up 'Til Dawn, Relay for Life, and other stuff.
I think my most enjoyable extracurricular was being a resident advisor for sophomores and freshmen (freshmen this year). They keep you on your toes (2:30 AM: "OMG the toilet just exploded, come quick!😱"), and it's amazing how much you learn about yourself and others through the experience.
Above all, have fun with your extracurriculars! Choose ones that bring you enjoyment, help you grow, and help you meet new people, not pad your resume.