Preserving cardboard blocks

[momsalive1]

My kids are older now, but in their youth they had a much loved set of cardboard boxes that became forts, race tracks, stages, whatever.  Its time for them to go, but I'd like to preserve a few of the smaller ones for them.  The cardboard is colored like a brick, and is about the half the thickness of a packing box.  I have no idea how to proceed, but I know you will!  Thanks!