At the gates to the Underworld, on the Arch above is written, "Abandon All Hope, Ye Who Enter Here." This is from Dante's "Inferno," where it is also written above the entrance to the gates of Hell.
Although he is credited, Robbie Kay doesn't actually make an appearance as Pan. He is heard in voice-over as Gold reads a letter from him at his shop.
When they enter Emma's house in the Underworld, the camera pans to a plush yellow bear and a plush tiger under a table. These are likely symbols of Winnie the Pooh and Tigger.