I came across this blog by George Allen Durkee...with regards to the question...What should I paint?...it says it all.
SOME THINGS TO KNOW ABOUT
By George Allen Durkee
If you don't have a juicy subject to paint it's not likely that you will be all that inspired to paint it. Turn that around. When you have a juicy subject the paint flows easily, and when the paint is flowing easily, anything is a juicy subject. It's no longer about the subject but about the process of painting.
The first rule, if there are any rules, is: don't try to make a painting out of THINGS. Not rocks and branches and moss and leaves. Instead, look for shapes, areas, places of color - light/dark, warm/cool, bright and not so bright, large and small, round and square. All of these "things" seem to be dualistic opposites, yet they join in a dance of wholeness. Look for the wholeness.
Look at the world. Look in different directions. Something catches your eye. See shapes as large and small, light and dark. Make picture borders with your hands. Frame the shapes that catch your eye. Sketch: nothing fancy, just small sketches to help see the design. Not just shapes in an unending landscape, but shapes held within an enclosure. Picture borders. Fit shapes within the borders, one big shape even larger than half the entire enclosed area. Subdivide the remaining shape into smaller shapes, one of them larger than half, and so on. Divide shapes into unequal parts.
When you begin painting, start with thin paint so you can develop and correct your work as you progress. Draw with lines or mass, whatever your whim. Outlines locate shapes. Once you're satisfied with the arrangement of main shapes, paint forms and let the outlines go.
Get a few key values on the canvas right away. Dark and light values rightly placed give you the range within which you will work. Use them to help gauge the major halftones of your composition. If you're fairly certain of your values, or you feel like taking a few risks, lay the paint on with finished brushstrokes. Otherwise, begin with thin paint, retaining the texture of the canvas and build progressively corrected color and texture later. With the thin-first-and-thicker-paint-later approach, build the painting through a series of stages, first covering the canvas tentatively, then correcting and refining while partially covering layer upon layer. This also gives a more scintillating paint surface provided you let the underlayers of paint show through successive brushwork.
As a general principle, it's best to forgo too much refinement and detail until the entire composition is settled and you know you've got the major shapes and values right. Next, refine the forms in an overall way so everything has a pleasing harmony. Then work on details. However, what often happens in the actual practice of landscape painting from life is that the light will be just perfect on a certain part of the landscape right now and you'll choose to nail it while it's there. At the same time you know that the light is changing on everything else. Even though you may choose to "paint ahead" or refine one part of the painting ahead of the rest, keep in mind that this can lead to discordant lighting where the direction of the light varies from one part of the painting to another. Something to pay attention to.
George Allen Durkee