Use a basic planetarium projector, available at toy stores, to create a universe on your son's walls. These battery-operated toys project images of the solar system. Use the projection as an invisible stencil to paint the images on the walls. Continue the space theme throughout the room with bedding and curtains.
Cover one wall with chalkboard paint, using a roller and paintbrush. Your son can do his own artwork on the wall and keep it up for as long as he wants. When he tires of it, he can wipe it off. You can also cover furniture with chalkboard paint.
Create a safari theme by painting animal prints on the walls. Find photos of zebras, tigers, leopards or giraffes. Use them as your guide. Paint the walls white, and add long, uneven lines in black to create a zebra print. Use a warm orange-brown base for a tiger print. For a leopard print, use an orange-brown base, and paint brown uneven circular shapes outlined in black. For a giraffe print, use a warm tan as the base, and paint large brown uneven circular shapes.
Paint paw prints on the wall. For a wildlife-inspired room, paint deer, elk or bear tracks. Look through books or on the Internet to find the tracks, and then draw them freehand on the wall. Alternatively, trace the images and create your own stencils to use on the wall.
Paint the walls light blue. Using masking tape, attach country or state map stencils to the wall. Draw around the stencils with a pencil, and then refer to an atlas to fill in the map. For example, use green to represent grassland, yellow for a desert and gray for mountains. Alternatively, add state capitals and points of interest, such as universities or ballparks.
Create a faux-cloud effect on the walls. Paint the walls a light sky blue. Then mix a solution of seven parts white satin paint with three parts water. Use a sponge to lightly dab the watered-down white paint on the walls in random cloud formations.
Paint the room using the colors of your son's favorite sports team, such as the New York Yankees or the Dallas Cowboys. Draw simple outlines of helmets, bats or balls, and fill them in with the appropriate colors.