Here are a few things I found on the internet that I loved! So easy and cheap! I have noticed through the years that if I involve the children in the "transformation" of materials - there is a deep appreciation and engagement in the items constructed. These are a few ideas that the children could be in the process of making them. Obviously, they can not cut the cardboard but they could help design and paint the item.
From Parents.com - they have a series of ideas for what you can make out of a box! I loved, loved, loved making things out of boxes as a young girl!
link: cardboard crafts
Dress up play! Make costumes for your dramatic play center out of throw aways! Great idea from RealSimple. com!
Link: Brown Bag Costume
Here is a kitchen set made out of a box! How cool is that? I found this on pinterest and did not find a site for instructions. If you know it - please let me know and I will attach it.