At one point, when Sam, Tucker, and Danny are at the entrance to the movie theater, there's a "Crash Nebula" arcade game in the background. Crash Nebula is one of Timmy Turner's favorite superheroes featured in The Fairly OddParents (2001) another show created by Butch Hartman.
The Whole show, in some way or another, revolves around names of ghosts or spirits. (The high school is called "Casper High" referring to Casper the friendly ghost.) Also, the town they live in is called Amity Park referring to the Amityville where ghosts/spirits caused a man to kill his family. Also, Danny's high school mascot is a raven, referring to Edgar Allan Poe's famous poem, 'The Raven'.
Another reference to ghosts being involved with the show is the main character Danny Fenton and his father, Jack Fenton. Danny and Jack were the father and aon's name in Stephen King's The Shining (1980).