Only do "research" during that critical summer if it will result in a publication. Get the PI who you will work under to *guarantee* this. I considered a number of interesting projects but picked the one where the PI told me that he would not take a med student on if he/she was not going to get a publication. Publications (for various reasons) are more important for resident selection than just "research"... Also, you probably want to steer clear of a basic science research project as those tend to take longer (in my experience)... Sign on to a clinical project that is as narrow in scope as you can possibly get. The simpler, the better. Try to get one in your field of interest, but if not it's no big deal. You can always say that you changed your mind, or that the project had a specific appeal to you for whatever reason. The main thing is that if you're gonna do research, make SURE you get a pub out of it. Otherwise you're better off spending the summer lounging poolside, or at the beach sipping on one of those drinks with an umbrella in it.
As for the overseas option... if you are interested in it, DEFINTELY do it. I took a year off between college and med school and spent most of it working overseas (in a non-medical capacity)... I have already had 3 interviewers ask me about that time. It shows that you have a sense of adventure, curious about other cultures, and it WILL make you stand out more than the applicant who did not go overseas. Granted, it may not be the one thing that gets you an interview, but it's something to chat about in the interview, and it gives the interviewers something more to remember you by.
ALSO, don't listen to people who criticize you because you want to make yourself "marketable". There is nothing wrong with doing a few things for the sake of "punching your ticket". Recently I had a conversation with a prominent surgeon who laughingly told me that there are some things she had done in her career just to "punch her ticket". Personally, I think it sucks but the reality is that some things matter to residency selection committees that don't necessarily matter that much to you. It's your choice.
Peace.