"The Coming Storm": Dancing skeletons
Slime the gravestones at the start of the level, then pull them down. Each of the skeletons from the graves will do a different dance.
Goo rocket
Look down with a male avatar, then shoot slime while you jump. It will shoot you up like a rocket.
Invisible character can see ectoplasm
When you get to the end of a section (for example, going down the stairs to hunt down the Grey Lady ghost, just after they brief you on the P.K.E. goggles), put on your P.K.E goggles and walk through the door or off the staircase. After the other Ghostbusters are finished speaking, you will notice you cannot see your character. You can only see through their eyes, and you can clearly see the trails of ectoplasm (the little colorful clouds of ghost goo) without the goggles on. To exit, return to the firehouse or wait until you get to the next part of the level.
Portal reference
When you get the scan for the Chef's cake it will say "Many researchers believe the cake is a hoax", which is a referrence to the "The Cake is a Lie" message found on walls in Portal.