You've heard of someone getting into Med with two F's? I hear to beware of anything lower than a B because that's when you start loosing competitiveness. But I guess it really does depend on the school. Just make sure you have really good grades in your science & math classes because Med school considers these two subjects in their own category called Science GPA (they also consider your overall cGpa, but science GPA is more important usually). High MCATs of course, and shadow doctors, and volunteer in hospitals. Get some good leadership spots in school too.
This is all assuming you're applying to Med.
In the end it doesn't matter if you failed two classes, get a B or whatever. What matters is your MCAT scores, which are indicative of how well you will do on your PASS 1, PASS 2 exams in medical school. As far as extracurricular activities go, just do what makes you happy, but make sure that you gain something out of it. You can volunteer in an organization for 3 years and if you have nothing to talk about, i.e. what you learned from it, then its useless.
80 percent of selection to medical school is MCAT and GPA. 20 percent is extracurricular activities.
If anyone needs medical school advising hit me up with a PM. I use to be a premed peer advisor for UCD