3D glasses (stereoscopic mode)
Collect all 64 Cubes then go through the Star Gate again in the 32 Cube room for the alternate ending. After the credits end, select the "New Game +" option again at the main menu. Progress progress to where you originally obtained the Fez and sunglasses to instead get a pair of red and blue 3D glasses. They will enable Stereoscopic mode. To disable this mode, enter the "Settings" menu.
Sunglasses (first person view)
Begin game play in New Game + mode and progress to where you originally obtained the Fez to instead get a pair of sunglasses. Click the Left Analog-stick to switch to first person view.