Hi
@Anna Ye ,
I just graduated from my first degree (Business Administration) and I am looking for a MFE too.
After my research on the MFE programs I concluded that your degree valuates a lot as a first degree, due to the fact that you have a lot of mathematics, statistics and so on. My advise will be to take as many math and statistics courses as possible, and if by any chance you have elective courses such as finance and economics, choose them too. If you don't have finance or economics at all, don't be disappointed, you can always create a profile in Coursera.org and get certificates in many financial courses online. All you need to do when the time comes, is to prove that you have some certified knowledge in Finance and basic economics. That's really not hard.
Concerning you choice for the summer, I would definately recommend you to do an internship. In this point of your life, real-life experiences in a work environment are way more valuable than another research. Don't get me wrong, research is good, but you have all the time in the world to research while you are studying
I was working as a freelance entrepreneur for 3 years and I learned so many things that school doesn't teach you, experiences and knowledge that matures you and makes you wiser.
One more advise: prepare, focus and plan your following years according to your choice and every academic decision you do, make sure it serves that purpose
that doesn't mean you shouldn't expand your knowledge more to other fields and widen your mind, but the core of your education should be oriented likewise.
That's my opinion of course, keep it in mind when you make your choice
I hope I helped.