Sometimes the best toys aren't toys. Last week I salvaged twenty or so cardboard boxes from a local hi-fi store for my children (aged 6 & 10). The boxes have been played with every day, and are now well and truly flattened. If it wasn't for the toy library where I work being packed-up for summer, I would borrow our double set of Makedo so that we could spend the days up to Christmas in some serious cardboard construction.
Makedo is simple and well-designed - a starter kit contains a plastic saw, socket screw driver, and 28 solid screws, which can join multiple sheets of cardboard. There's a spike on the end of the saw for pre-punching screw holes and perforating folds.
When I brought the set home previously, my eldest jumped right in, making a car/house with a closing door. Using the screws and driver was also a nice challenge for my six-year-old.
The hardest part about this toy being in a toy library collection, is that to return the toy, you have to break the constructions down. But the cardboard can still be kept for further imaginative play.
Take a look at the Makedo website here.