This Instructable is a very simple version of a toy playset that I have recently seen around a lot. The idea behind it is that it provides a relatively blank canvas for a child's imagination. A child can play with whatever toys he wants on it and it can be whatever he wants it to be (i.e. a house, a tree fort, whatever).
The version I made is pretty sparse but you can add ladders, stairs, swings, pulleys, pretty much anything you want. Right now my son uses it for his peg people