Cut a log into lengths approximately 20 inches in height for chairs. Sand both ends of each chair until they are smooth and level. Cut a flat surface longitudinally on each log to attach the back of the chair.
Cut boards approximately 40 inches in length and 15 inches in width for the backs of the chairs. Nail the boards to the flat sides of the logs, and sand away any rough edges. Paint or stain the chairs if you so desire. Allow the chairs to dry on the ground before moving them into the treehouse.
Nail a rectangular or circular piece of plywood to a 20-inch-tall log for a table. Stain or paint the table.
Cut a tablecloth out of a colorful shower curtain. The waterproof material won't mildew in rainy weather. Use the leftover fabric to make curtains for the treehouse windows.
Nail one crate, open side facing the room, to the floor and the walls in the corner of the treehouse. Put wood glue on top of the first crate and stack another crate on top. Nail the second crate to the wall and repeat until the shelves reach your desired height.