i went to costa rica and nicaragua 2 years ago and had a wonderful experience. i plan to return sometime this year. i'd highly recommend Global Medical Training: http://www.gmtonline.org/
i go to med school in san antonio - we have monthly weekend trips to various places around mexico. i haven't been on a long trip to exotic locations but i found them to be really cool from an educational standpoint as well as getting the opportunity to have hands-on experience.