Most definately research and if you stick with a project long enough then authorship, no matter where you fit into the paper( 1st author, second, etc.) is a MAJOR plus. In addition, a lot of the top medical schools offer summer research programs so if you identify a top-tier school that you really like and you are early in your undergrad education then start looking into those summer programs..these programs are meant to be recruitment tools for the school so if you get accepted into one of their summer programs then it is a major shoe in for acceptance into grad/med school, and as well you will get to meet a lot of the faculty/admissions committee while you are working in their labs during the summer. This is only suggestions if you are actually interested in the research side of medicine, if not then look into other programs in your area of interest that will make you stand out as an applicant but make sure you enjoy those experiences and it will come through in your personal statement, interviews, etc..