Grim Fandango reference - In Le Chuck's lair, look closely at the pillars made from skulls. One of the faces is that of Manny Calavera's head.

Maniac Mansion 2 reference - Play the game in Special Style mode, and go to the monkey head entrance. You can see a Purple Tentacle idol.

Sam And Max series reference - Play the game in Classic Style mode, and go to the monkey head entrance. You can see Sam and Max idols.

Accomplish the indicated achievement to get the corresponding number of Gamerscore points:

Determined (10 points): Completed the game without using any hints.
Escape Artist (10 points): Escaped from the monkey island cannibals hut 5 times.
Guybrush Kicks Butt (45 points): Completed Last Part: Guybrush Kicks Butt. Guybrush stopped LeChuck from marrying Elaine Marley.
Human Cannonball (10 points): Finished the entire game in record time. We hope you were wearing a helmet!
Marooned Everyone (10 points): Re-marooned Herman Toothrot and marooned Carla, Otis and Meathook on Monkey Island.
Old School (5 points): Experienced the original, classic adventure game, as it was in the early 90s.
RecordKeeper (10 points): Collected all of the notes and memos sent between Herman Toothrot, LeChuck and the Cannibals.
Sharp Tongue (15 points): Guybrush learned all of the insults, and is the wittiest/deadliest pirate what ever swung a sword.
Ten Minutes Later... (10 points): Guybrush can hold his breath for 10 minutes, but not any longer than that.
The Journey (25 points): Complete Part Two: The Journey. Guybrush used a voodoo spell to travel to Monkey Island.
The Three Trials (25 points): Completed Part One: The Three Trials. Guybrush proved himself worthy to be a pirate.
Under Monkey Island (25 points): Completed Part Three: Under Monkey Island. Guybrush traveled into the Monkey Head and got the root.