When you have collected ALL 64 Cubes go through the Star Gate again in the 32 Cube room to get an alternate ending. When the credits have ended select the 'New Game +' option again at 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 will get a pair of red/blue 3D glasses which will automatically enable Stereoscopic mode. This feature can be disabled at the 'Settings' menu if you do not want to use it.
Play New Game + mode and when you reach the point in the story where you recieve the Fez on the first play through you will now get a pair of sunglasses instead. The sunglasses allow you to switch to first person view when you click the Left analog-stick.