One of the best ways to stand out as a medical student applicant is by participating in research studies or performing your own research. Of course, presenting or being published is usually what people are after, and being part of a major university study increases your chances of being published or swept off to a fancy conference. Accepted.com has just posted an exhaustive list of some of the best research programs available to undergraduate students. These programs are all 6-12 weeks and many provide a nice stipend and/or living arrangements.
Remember, anything you do above and beyond your average undergrad student is going to set you apart from other applicants--not just for medical school but also residency--and is usually well worth the effort!