While working at Barney's New York, Jack's boss Dorleen tells he and Grace that she is in a pinch because her window dresser "Simon" has suddenly up and quit and she needs someone to design a Christmas window. In real life Barney's New York's main window dresser is Simon Doonan.
When Robert lifts Will as they're leaving the dance studio, you can see the top of the set in the upper left corner
After seeing the display window that Grace had decorated Jack says "Mary-Kate and Ashley, it's beautiful." Eric McCormack who plays Will Truman played the father of Mary-Kate and Ashley Olsen's characters in the movie Double, Double, Toil and Trouble.