Lay soft, green or brown wall-to-wall carpeting to act as the forest floor. The carpeting should be thick and soft and can have a subtle surface texture to add visual interest.
Paint three walls a dark green and add some silver and gray tones to the walls by dabbing them on with pieces of sponge. Smear or buff the sponging with a dry soft cloth. Let the wall paint dry completely. Stain the trim around the windows and doors with a dark stain to resemble dark wood grain patterns.
Paint a forest scene mural on one wall that includes trees, birds, squirrels and deer. Add a moon in the phase of your choice, and highlight the trees in silver paint to represent the moonlight. Paint the ceiling a dark midnight blue, and let it dry. Add some stars by gluing bits of silver tinsel to the ceiling. Tilt lamp shades upward slightly so the light catches the tinsel stars and makes them sparkle.
Place dark wood furniture such as a full height storage bench along one wall. Arrange dark brown or black leather couches or love seats around sunken carpeted steps that lead to a flickering fireplace. Add large gray or brown ottomans or beanbags scattered around the room to look like boulders.
Hang long, thick, wooly green drapes that drop to the floor on all of the windows. Add a rain stick in the corner of the room for creative play if you are using a rainforest theme. Add natural elements such as birds' nests on shelves or a table and pine cones in a wooden bowl.