One way of sorting out what stays and what goes is to make a study in which we reduce each of the major shapes to an area of a single value with little or no texture.
In this scene the sky, the mountains and the cactus resolve very nicely into big shapes of light, middle and dark, respectively. Only the ground plain requires simplification. Since it comprises several secondary shapes that vary in value it is tricky to assign it just one.
Thinking of the illusion of space, is there a way we could simplify the busy territory between the cactus and the mountain that would contribute to a feeling of depth? What if it were lighter than the foreground (cactus), and darker than the background (mountain)?
Try to keep the overall number of values to 5 or fewer. Squinting will help keep it simple. Try it with this barn scene. How dark is the grass compared to the new roof? How about compared to the sky?
Here are a couple more images that are made up of a few shapes that separate from each other due to value differences. Working with one of these or one of your own, make a study that is deliberately over-simplified. Stand back from the finished study and ask yourself, "Where do I want greater subtlety or specificity? Where is an over-simplified treatment sufficient?"
Move on to a proper painting that makes use of this exercise. Remember to "bracket" your values by finding something in the scene that is darker than the shape in question and something that is lighter. The vegetation in the photo below, for example, is darker than the sunlit walls but lighter than the shadows.