In "Dreamscapers" there was a broken swing set. It appears again in "The Last Mabelcorn". The swing set most likely represents Stan and Ford's relationship. At the end of "Not What He seems" we see the backs of the "original mystery twins". Swinging slowly back and forth on a beach. This being a happy memory. Also in "A Tale of Two Stans" when teenage Stan and Ford are talking they are on a swing set. Notice how in both their minds they have the swing set and its broken. Representing their broken relationship, it is a constant thing in their minds. This probably stating that they will be friendly again soon. But I noticed the swing set when "Dreamscapers" came out! Back when the Stan twin theory was around and buzzing. This only made me believe the Theory even more! (BEFORE NOT WHAT HE SEEMS CAME OUT). Overall, this swing set could symbolize lots of things. Whose to say that it won't be in Mabel's mindscape or bubble? She is not happy with dipper and thinks he will be gone forever, leaving her alone.