During the month of December 2015 Delta is offering all Delta American Express card holders the chance for two “top shelf” drinks free of charge per visit.
Delta has recently been upgrading the complementary choices, both food as well as drinks, but this is a way for you to try some of their normally up-charge drinks at no additional charge.
If you happen to have the Delta Reserve card personal or business from American Express, like I do, then entrance for yourself is complimentary as well as the two free drinks during the promotional period.
Delta does not require you to actually scan your card for the free drinks so I imagine if you went to more than one club during your layover you could utilize this benefit at additional Sky Clubs. – Rene