I'm not sure how to link my Foursquare account to my credit card, but this Christmas deal by Harvey Nichols seems like a great idea. Basically, it's 25 quid for free just for a bit of fiddling about on the internet.
They're also doing some fun stuff for Amex users over Christmas, albeit more traditionally. Anyone using their British Airways Amercian Express at the checkout is invited to pick a card, which will offer between 15% and 50% off the total bill.
Christmas is all about the giving, right? More of this, please.