Jessica Simpson visits Macy's South Coast Plaza in support of the Jessica Simpson and Jessica Simpson Girls Collections on November 10, 2012 in Costa Mesa, California. Credit: Jamie McCarthy/Getty Images

Jessica Simpson knows how to make the most of a cheat day.

The Weight Watchers spokeswoman -- expecting baby no. 2 -- went all-out at a December 2 birthday lunch for BFF CaCee Cobb, 35.

"Jessica ordered a grilled cheese sandwich with onion rings," a fellow diner at the Polo Lounge in Beverly Hills tells Us Weekly.

As her friends toasted Cobb with cocktails, the mom to Maxwell, 7 months -- who has yet to confirm her pregnancy -- waved off her usual white sangria.

"Guess what?" she told her server, with a smile. "I'm drinking ginger ale!"

In anticipation of the baby's arrival, Simpson, 32, and fiance Eric Johnson, 33, are in nesting mode: A source says they just made an offer on a house in L.A.'s luxe Hidden Hills!