Box Orientation
When you decorate or outfit a box as a play spaceship, start by deciding what orientation you want the box to have. Having the opening at the bottom is generally a bad idea, because it can be unsafe and hard to monitor -- even if you cut lots of portholes. Open side up is the most natural orientation, but the child may have difficulties getting in and out, especially if the box is large. A sideways configuration allows easy access and you can set up the rocket with the opening either as the front or a cutaway wall.
Outer Decorations
The best spaceships have impressive exteriors. You can fashion a point, wings and tail out of the flaps of the box or out of cardboard cut from another box. Cut triangular wings or tail fins and use sturdy tape to attach them to the exterior of the box. Fold a large piece in half and tape it to the front to form the nose of the spacecraft. You can even make flames for the back out of stiff red, orange and yellow construction paper. Cut a porthole or two for the pilot to look out of on missions.
Inner Fittings
If the box is big enough, you can include a cardboard control panel and other accoutrements into the spaceship. To create a control stick, push the end of an umbrella through a small hole near the bottom of the front wall then loop a few thick rubber bands around the handle and staple them to the side of the box. Put a small cushion or pillow in as the pilot's seat and decorate the walls with pictures of buttons, gauges and readouts, with a star field drawn inside the windshield area.
Accessories
You can use the extra pieces of cardboard that are left over to create play accessories. Cut spare box flaps into smaller rocket shapes that your child can play with while running around. Having a small replica of the big rocket expands the potential for imaginary adventures. Circles cut out of portholes can become moons and planets with a little bit of crayon work. Miscellaneous bits can become asteroids, space jewels or aliens for your child to discover.