Accept and then possibly renege later, or ask for extension?

I'm in a sort of unique situation, I go to a super non-target, i.e. no big companies much less banks recruit there. I'm a junior studying math and minoring in accounting, and because of the above I knew I couldn't rely 100% on getting any sort of banking offer so I had to apply for other positions as well. I've looked into becoming an actuary (haven't taken any exams yet) and submitted applications for summer programs with various firms, in addition to the ones I submitted for banking. I recently interviewed at one of these, a Fortune 100 HC Company. I found out today I got the offer, and I found out yesterday I got a superday for a BB, (not IB, but lower level FO). I'd prefer the BB, but obviously I don't yet have the offer, and even though I have a good GPA etc., coming from my school I don't want to take any chances on saying no to one offer and then not getting another for next Summer.

My question is basically whether I should go ahead and accept the first offer, go to the superday, and if I do get the offer there, renege on the first one. Or, whether I should simply ask for an extension to decide on my other offer and see what happens with the BB. The superday is in a week, and I've heard it takes around a week to know whether you got the offer or not, so I'm worried the first company won't want to give me two weeks to decide. The other thing I'm worried about is getting to the superday and being asked if I have any other offers or if I'm in the process with anyone else; I've heard you can get dinged if they find out you aren't only interested in banking/finance. I thought I could spin it like I had to diversify my options coming from a school where banks don't recruit much, but I worry this could make it sound like I have a chip on my shoulder about it. Any advice would be very helpful, thanks guys.