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.
Bonus options
Complete the indicated task to unlock the corresponding entry at the "Options" menu.
Equipment Strength Upgrade: Collect all 103 art illustrations for Tobin's Spirit Guide.
Faster Health Recovery: Collect 50% of the art illustrations for Tobin's Spirit Guide.
Gozerian Rookie Outfit: Complete the game 100% under 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: Complete the game with all 103 art illustrations collected and scan all 103 of the ghosts and scannable objects for Tobin's Spirit Guide.
No Equipment Overheat: Scan all 103 the ghosts or scannable objects for Tobin's Spirit Guide.
Goo rocket
Look down with a male avatar, then shoot slime while you jump. It will shoot you up like a rocket.
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.