I think I should build a playground full of doors. Doors and drawers. Cabinet doors, bedroom doors, sliding doors, pocket doors, folding doors, rotating doors. Big heavy drawers that roll with a hollow chunking sound in and out. Little tiny ones that make a satisfying click. All sorts of knobs and latches and locks. And not just doors on walls, but doors in frames that stand upright that you can go through. Again and again and again.
Ooooh. And real working telephones and remote controls. None of this fake toy business with a canned loop of recorded sayings. They know the real thing when they see it, and nothing else will do. A water fountain, of course, or even just a pipe or two providing a steady drip of water to play in. All issues of water preservation aside, you know they'd love it.
To make it truly paradisiacal, the playground would also have lots of plastic boxes and baskets. Okay, okay, to make it truly paradisiacal, the boxes and baskets would have to be full of toys and books for the children to dump, on the way in and out of the doors, and a playground attendant to refill them, ready for the next pass-thru, but that might get expensive so lets just stick with the boxes. Unless someone can cheaply produce a holographic system wherein we could fill the boxes and drawers and cupboards with computer generated images of toys, household goods and dishes that a child could sweep out, with full sound effects, but they'd just disappear as they hit the ground. Mattel, Nintendo, Microsoft, are you listening? This could make you the toy giant of the millenium.