We're not sure if you can really buy this thing (there's a seemingly unfinished website and a similarly unfinished shopping page), but the Rolobox sounds like a whole loads of fun for young kids. A bolt-on kit, any kid can use it to turn erstwhile useless cardboard boxes into a brand new toy vehicle.
Remember the Makedo, which lets you employ a three-component construction set to turn junk materials into a playable toy? Well, this is like a specialized version of it, letting you fashion that hauling box into a large wagon and that shoebox into a fake pickup truck (if you've got the imagination for it).
Each Rolobox kit includes four wheels with corresponding brackets and screws, along with a bolt-on tail that you can use to haul your new box cart with. All parts are made from plastic, with no pointed ends and sharp edges to keep playtime safe. You'll have to punch holes on the cardboard to get the screws in, though, so you might want to do it yourself if you don't like the kids puncturing holes by themselves.
After your kid tires of a box, the parts can easily be removed for affixing to another one, so it's a perpetually usable kit, provided they don't get too bored of playing with carts. Buy a few of these and you can turn up a veritable train of cardboard carts, too, making for some serious fun.
The Rolobox is aimed at kids between 3 and 7, priced at $14. Though, like we said, we're not sure how (or where) you're supposed to get it.