What is the easiest way of making a beautiful and easy landscape for a sixth grader?
If the landscape must be two-dimensional, colored pencil would probably be the easiest choice. Another factor to consider would be the size you want for your finished landscape.
If allowed, a better representation of a landscape would be to make a three-dimensional model of the features you want to include. Paper mache is easy to make (see http://www.dltk-kids.com/type/how_to_paper_mache.htm for directions); newsprint can be used for creating the base of mountains, plateaus, plains and then topped with a layer of white paper; it's easy to shape valleys or lakes and rivers while the material is wet; paint can be added after the paper mache is dry to visualize grass, water, and bare rock; and it's fun!
Modeling clay could be used to make another type of three-dimensional model of landscape but would cost more than paper mache.