How to spend summer productively?

Ok, so I currently attend a non-ivy, but highly ranked public university in the US. I'm double majoring in math and statistics, but am a little behind with my stats courses because I dabbled around with engineering in my freshman year. However, I'm certain that I want to get into banking, and intend to go into my math and stats courses full-steam ahead in my sophomore year. (next year).

My question for now, however, is what I should be doing to improve my resume and acquire useful skills. I plan on taking the GMAT this summer, and I'm confident I can get a high score (I'm good at standardized tests). However, what besides this would be a useful activity to do?

I was thinking of working at a local restaurant like Panera or something, just to show I'm willing to work hard and that I'm not some entitled SOB, but does this look good at all on a resume? I would also like to pick up some programming languages, and maybe get some real world experience applying them, but i'm not sure where to start.

Any general tips would be greatly appreciated, since I would ideally like to maximize all my time during the summer so I can put myself in a good position applying for internships next year.

Thanks in advance, I hope my first post is informative enough...I can post some stats from high school if you guys need any indication of what type of student I am.