I recommend you take the GMAT test again. With the GMAT, understand that your score is self-reported on your application. This means that schools will be focusing on your highest score- there isn't a downside to taking the test a second time. In fact, we encourage candidates to do this. The average GMAT score at Stanford is a fair amount higher than 680.
If you would like, here are some free GMAT tips: http://www.stacyblackman.com/tag/gmat/
Conrad and the Stacy Blackman
Interested in a free 30 minute consultation with the Stacy Blackman Team? Sign up here: http://stacyblackman.com/contact
Stacy Blackman | Stacy Blackman Consulting Inc | http://www.StacyBlackman.com | +1 323.934.3936
MBA blogger, US News and Author, The MBA Application Roadmap