We quite often make random stuff by glueing together what's in the recycling bin. The kids help with putting it together and then they paint and decorate it and when it's dry, play with it. When the creation loses it's appeal they can pull it appart and put all the bits in the recycling bin outside.
Here's the castle we made and the bizzare game of balancing teaset items on it that was popular for quite some time. Take a moment to notice the toy lion that looks like he's taking a leak on one of the turrets. Nice.
I think this was made up of a couple of cereal boxes, a pancake mix box, a kfc popcorn chicken box (mmm dirty), some yoghurt pots, some toilet roll tubes, some paper plates and a few plastic forks, oh and a couple of sandwich bag boxes.