Just to clarify a few things, you have 14 calendar days from receipt of delivery to file for a return, and another 14 days to ship it to Apple to get the credit.
You are outside the window, but selling on Craigslist or eBay is an option.
I also started with a Space Grey and realized after a week with it that I really wanted the nicer SS. I made this decision last weekend, with a couple days left in my return window, so I put in a return request. I happened to preorder a SS on May 3, anticipating that I might want to upgrade (I knew I could always cancel the order if I didn't). The SS arrives tomorrow and that's when I'll put my SG back in the box and ship it to Apple.
I considered selling my SG on eBay for a profit, but the selling prices are quickly dropping given that Apple is catching up with orders and the impatient people have already mostly figured out how to get a watch. There is still a profit to be made but minus shipping and eBay selling fees, it was not enough for me to bother. I don't like selling on Craigslist so I decided to just return it to Apple.