Something that gets passed around in DH's family is this great set of Melissa and Doug cardboard bricks. They are big enough to make towers and forts out of, and they are super durable, but they are light enough that they wouldn't really hurt anyone or do any damage.
http://www.gummylump.com/front/f_product.php?id=1485
The kids love them. Heck, DH loves them. They've been passed on to us, and now that I know where to buy them, I might just get some more.....