- garage - my son loves using boxes as a garage for his matchbox cards, you can also use them to build parking structures
- doll house - don't forget the girls, cardboard can make the house and everything in it
- put under those older cars you have (also a money saver!) in case it drips a little to keep your garage looking nice
- paint them! great for crafts
- large boxes can be a play house for the kids to play in, you (adult) can cut out windows for them
- organize - cut slits and put together as a grid to organize a drawer into compartments
- burn it in your fireplace or outside fire pit
And when you are all done doing that, then recycle the cardboard.