Join SDN on December 7th at 6:00 PM Eastern as we host Andrew Paulson of StudentLoanAdvice.com for an AMA webinar. He'll be answering your questions about how to best manage your student loans. Register now!
Do a search, lot's of lengthy threads on this already.
Short answer is some people take time off or travel for leisure but most people do something with residency in mind. If you're going into academic medicine or specialties like surgery, onc, etc. Research is a pretty big deal during this time. The people that I know that are geared towards going into primary care either did research as well or took time to volunteer in clinics/community organizations or just did whatever they wanted. But it's definitely a good time to improve your CV for residency seeing that it's one of the only free summers you have from here on out.
If you are going into a more difficult residency or really want to do something productive towards your resume, do research/trip for medicine-related things. If you're not as ambitious, take it off and relax, go on vacation.
If you do research, it does not mean you can't take a week or two and still fit in a vacation in there somewhere.