Over at This Old House there's a fun idea to build a fort from simple fencing panels. It allows for easy assembly and there are only a few cuts to the whole project (that the hardware store could make for you). Just a few screws and you're in business.
If you don't have a yard, it could easily be set up inside with hinges instead with a few safety measures to help keep things upright. It could be flattened and slid under a bed or placed at the back of a closet to keep it out of the way.
(Image: Wendell T. Webber/This Old House)