I know this is 4 years old, but here are some
•Cabby says: “...and I ain’t afraid of no ghosts!” which is a reference to the original theme song
•Martin Heiss says : “Casper...?” which may be a reference to Raymond Stantz’s cameo in Casper 1995
•The green ghost emitting from the food stand is a reference to the Onionhead in both his appetite and appearance
•A bust of Harold Ramis’ head can be seen outside of Erin’s office in the University
•A Stay Puft balloon can be seen at the parade
•Many of the events in the movie also have similar counterparts in the Ghostbusters Video Game