Lay the cardboard box on the side so that the opening faces you. Paint the inside black or a very dark blue. Add a few stars and galaxies with white paint.
Sort the plastic foam balls into four sizes. The largest ball is the sun. The next largest are Jupiter and Saturn, followed by Uranus and Neptune, and then Mercury, Venus, Earth, Mars and Pluto.
Paint the plastic foam balls with tempera paints. The sun is yellow. Mercury is brown. Venus, Jupiter and Saturn are brownish-yellow. Mars is red. Earth, Neptune, and Uranus are blue. Pluto is black.
Cut five rings out of poster board. Four rings will make the planetary rings for Jupiter, Saturn, Uranus and Neptune. The fifth ring is the asteroid belt. It must be large enough to fit between the orbits of Mars and Jupiter
Glue the planetary rings to Jupiter, Saturn, Uranus and Neptune. Draw asteroids on the asteroid belt with felt markers while the glue dries.
Glue the sun and the planets to the tips of straws. While the glue dries, cut two pieces of fishing line that are the length of the box opening's width. Punch two holes with your scissors into the center of your display box's top.
Drop the pieces of fishing line through the holes so that all ends fall to the same height. Tie off the fishing lines with a knot at the display's ceiling so they won't slide around later.
Glue the straws supporting the sun and the planets to the display's bottom. Place the sun in the center, then: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Tie the ends of the fishing line to the asteroid belt's quarter points.