Legos. In black bags. A person must select a Lego from the bag and place it onto the structure, and then the next person does the same.

( note, this game is called The Awful Tower to me and my own)

Ah, that's a fun one.

One of my colleagues suggested getting a bunch of oddments like pipe cleaners, fuzz, and little Army guy type figurines and having people draw them out of a bag, or giving them a small grab-bag full, and then letting them go nuts with whatever they want to do to them. We'd provide them a little base or pedestal piece to build on.

How about rolls of aluminum foil for people to squish into foil sculptures?

Ooh, love it.

What about Cornell type boxes? You could have shoe boxes or small cardboard or even wooden boxes (Michaels has them for like $1 each) and then all you need are a bunch of odd things - bits of computers might be good and gears and spools and stuff like that - magazines to cut up and glue and voila, assemblage. The bonus is they're easy to carry with them when they leave the table.