Unlock the following bonus options at the "Options" menu by performing the corresponding tasks:

OPTION - HOW TO UNLOCK
Equipment Strength Upgrade - Collect all 103 illustrations for Tobin's Spirit Guide.
Faster Health Recovery - Collect 50% of the illustrations for Tobin's Spirit Guide.
Gozerian Rookie Outfit - Beat the game on the Gozerian difficulty. This outfit will allow you to be immune to sliming.
Increased Scanning Speed - Scan 50% of the ghosts and scannable objects for Tobin's Spirit Guide.
Invulnerability - Beat the game with all 103 illustrations collected and scan all 103 ghosts and scannable objects.
No Equipment Overheat - Scan all 103 ghosts and scannable objects for Tobin's Spirit Guide.

In Ghostbusters HQ, there's a table upstairs with a phone. Interact with the message machine to the left of it to listen to a variety of voicemail. There is new voicemail each time you go to Ghostbusters HQ.

Look down with a male avatar, then shoot slime while you jump. It will shoot you up like a rocket.

When you get the scan for the Chef's cake it will say "Many researchers believe the cake is a hoax", clearly referencing the "The Cake is a Lie" message found on walls in Portal.