I am such a huge advocate for imaginative play.  I think children need a good daily dose until at least 9 years of age.  There is a developmental unfoldment of the imagination that provides the ground work for higher order thinking.   Imaginative play provides nourishment for the frontal lobes.  In reality, imaginative play is a very academic activity – we just don't see the results directly.

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.
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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

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Dress up play!  Make costumes for your dramatic play center out of throw aways! Great idea from RealSimple. com!
Link: Brown Bag Costume

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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.