Obviously fake foam-plastic rocks at the entrance to the tunnel as Steve removes them, since they are all the same color, all of semi-spherical shape, and have similar rounded-edged contours.
The puddle of water that Steve drinks from and gets poisoned is unnatural-looking; the sand around it is visibly dampened from seepage. Obviously just a puddle formed from water poured over a hollow dug in the sandy soil.
Around the 43min mark when Steve enters the cave he makes a small opening by removing some rocks, when he enters the cave he puts his hand down on some rock to support his weight, you can see the supposed rock deform under his weight, obviously made of some kind of foam or grey coloured soft material, rock doesn't compress and go back into shape.
After the cave-in that supposedly seals the tunnel's entrance, a strong natural-white light can be seen coming from the right side of the frame, obviously caused by daylight or a powerful spectrum-balanced movie light.