The "inside-out gas" that turns the family inside out is a reference to a 1962 radio drama album called "Drop Dead!" by radio performer Arch Oboler. In the segment called "The Dark", a man inhales a gas which turns him inside out.
The various alternative houses that Homer creates via his time-travel escapades: The Flintstones (1960) house The Original McDonalds restaurant which is located in Downey California, Shoe house from Mother Goose, Egyptian Sphinx, and an underwater Simpson's house.
There is a deleted scene on the DVD that takes place during "The Shinning" in which Bart is riding his skateboard through the halls, turns a corner and sees Sherri and Terri who sing "Your father's gonna kill you!". He runs away and turns another corner and sees Patty and Selma who tells him "It's true, ya know".
The trivia items below may give away important plot points.
Groundskeeper Willie is killed in all three vignettes while trying to rescue someone. All three happen in the same manner; he gets an ax in his back. In the first vignette he's killed by Homer, the second by Maggie and the third by Principal Skinner.