Hold E and click at the following parts of the game to view hidden secrets.

Start: Click on the suit's faceplate.
Shangri-la: When one screen away from the pilgrim, click towards the pilgrim. Then, after you move, look left to see a can of Mountain Dew on the white ornament.
El Dorado: Go back to the central temple after you have been decked down the shaft by Khan in Shangri-La. Click the ventilation shaft's zoom magnifier to see the center of the earth.
Eldorado: Click while walking toward the Shaman's house. A cat will appear on the side of the balloon.