What are everyone's thoughts on the best extracurricular sport that doesn't involve a team/varsity ex: marathon running etc...
At the moment I'm a bodybuilder, but I'll never get to a level where I would compete, so I'm looking for alternatives that would look extra-ordinary on an application.
Marathons/triathlons aren't that unusual among MBA students so I doubt they'd really stand out on a resume. You said you're looking for something that doesn't involve a team, but do you have any interest in playing rugby? Despite its barbaric-looking nature, plenty of corporate-types play well into their 40s and 50s so it's got a big upside on the networking side in addition to being a very social sport that's a lot of fun to play. And you could potentially take on an officer role within the club or head up a community service effort or something like that, which wouldn't hurt on an application.
If you're near a big city they should have a handful of club teams at all experience levels so you could start playing even if you've never watched a match before.