asdf
do you mean like building a wooden stool or making a chest or something?

sandra_panda
I had a friend that made a mandolin out of a cardboard box. She stained the outside with a wood varnish, and it had a pretty good sound. I would also suggest you try making simple kinds of furniture with it. How about a very modern looking coffee table? There is a famous lounge chair made of cardboard by an architect.

Harriet
I love the idea of the furniture and the mandolin. Re making the boxes more solid: it may be easiest in the end to take the box apart into a 'cross' shape (ie unfolded to flat) and then glue more cardboard onto the outside surfaces, matching each square or rectangle of the original box. When you reassemble it, it should be more solid. But then there will still be finishing work to do to stop it looking 'cardboard-scruffy'. For a smaller box, best to make it up from smaller, thick 'blocks' of cardboard made by gluing layers. One thing I'd say is that if you are using corrugated cardboard, work out which way the corrugations are going and glue layers of cardboard with the corrugations at 90 degrees. (Use thin wood glue or PVA). By alternating the direction they go in, you'll create more solid structures and it'll prevent the cardboard from folding up so easily when there's any stress on it. If you layer enough pieces together you can end up with a solid block which when dried can be roughly carved. You could even saw it into slices and use the textured slices to make something with. The main thing that makes cardboard look good is covering it well, careful edging and preventing creases. Applying precisely cut-out collage to layered cardboard and then varnishing it is good as a surface decoration. Metallic paint, or spray enamel like car paint or Hammerite will give it a heavy look. (You may need several coats because the first layer(s) will soak in. Make sure you're in a well-ventilated space). The thing that makes the difference to a cardboard structure is what you do to the sides and corners. It may be useful to paper the final shape with tissue or even cover it with papier-maché before applying decoration. Consider buying some cheap house-build filling material (like woodfill) to tidy up and solidify edges and corners. It can be sanded smooth afterwards. Then, regular house-paint as an undercoat will make the construction look more solid and crafted before you begin final decoration. Decorater's tricks like trompe-l'oeil marble or wood effects using house paints would be effective too. Hope this helps. Have fun!