I happened to walk by one of the Shake Shacks participating in the #DeltaAmexPerks Shake-Over promo where each cardmember and a guest were able to select one hot item (burger, chicken sandwich, flat-top dog), fries, and one cold item (shake, frozen custard, concrete or fountain drink) for free by showing their Delta SkyMiles card. The line literally wrapped around the block. People in line were asking others who walked by if they wanted to join them in line to take advantage of the guest perks. Delta had flight attendants on hand who were interacting with cardmembers, while AMEX was looking to sign people up for their Delta cards.
You can check out the limited time increased Delta sign-up offers that are good through July 6 here.