That's a great, great question! Thank you for starting this. There is obviously a question of "where" but also a bigger one of "what" you would cover.
I think you absolutely want to get deeper in the area that interests you, especially if you are switching industries and you are aspiring to jump into finance, IB, PE, or whatever else - I would pursue courses in that area/dimension just to get your feet wet and help with recruiting but also to get a real hard look at the subject/area/topic/industry and see if that's what you truly passionate about over a long term or if that's a fad.
However, my suggestion for you is a lot harder than taking an online class while tired after work. Nobody will care what MOOCs you took during recruiting interviews and during networking events. What matters is getting your foot in the door and standing out. That means - polishing and perfecting your SPEAKING skills, Interviewing, and Presentation skills. These are slightly perishable skills but you can use them anywhere in your life and career. You can use them at your cousin's wedding, at a party, or in an accounting conference meeting talking about the world's most boring things (no offense).
Another skill i feel is in high demand right now is Project Management. If you can get PMP or some kind of a project management skillset. I see this being appreciated and valued by many TECH and other employers. If you can get the PMP courses/training/etc and start applying it in your current job, that would go much further than taking it during your summer before bschool.
Bottom line - think about what would help you ace your interviews and your job recruiting. That's all that matters in BSchool. Your accounting, OB, or Marketing classes likely will have as much of an impact on your BSChool experience as better recruiting and interviewing skills.