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Red/Blue 3D Glasses
Collect all 64 Cubes and then go back through the Star Gate in the 32 Cube room for a different ending. After the credits, select New Game + one again from the main menu. When you reach the point in the story where you received the Fez and the Sunglasses the first two times you now get a pair of red/blue 3D glasses instead. These automatically enable Stereoscopic Mode, which can then be disabled in the Settings menu should you not want to use it.
First Person View Sunglasses
Complete the game and select New Game + from the main menu. When you reach the point in the story where you recieved the Fez on the first play through you will get a pair of sunglasses instead. These allow you to view the world in first person by pressing in the left analog stick.
First Person View Sunglasses - Play New Game+ and reach the point where you recieve the Fez
In New Game +, tap Up, Up, Up, Up+A at any point to fly.
If you come across a purple post with a square hole at the top, there is a Tetris code somewhere nearby. Switch to first-person mode to see it. They are usually on the ground.
The Legend of Zelda References
The poster in Gomez' bedroom above his drumset. (NES Legend of Zelda title screen)
When you first leave town, your polygonal friend says "Hey, Listen!". (Navi in The Legend of Zelda: Ocarina of Time)
A numbers game (10) Find the Counting Cube artifact.
Achievement unlocked (15) RTRTLTRTRTLTLTLT
Cryptographer (15) Break the code.
Equal and opposite (15) Find an Anti-Cube.
Get a cube (10) Find your very first cube shard.
Haikus not epics (10) Find the Tome artifact.
Hexahedronaut (25) Collect all 32 Golden Cubes and 32 Anti-Cubes.
Kill screen (25) Get to the end.
Mightier than the sword (10) Find the Writing Cube artifact.
Phrenologist (10) Find the Skull artifact.
Sum total (40) Find ALL collectibles.
Warp zone (15) Reactivate all 5 warp gates.