When you are in the hotel ballroom with Slimer, press [Action] by the piano to sometimes hear the same notes that Peter played in Dana's apartment in the first movie.

In the Sedgewick Hotel level, you will pass room 1204 while hunting for Slimer. Inside you can hear the original Ghostbusters commercial from the movie. This is also a reference to Dana Barrett, who had a habit of leaving her television turned on too loud.