Just wet the ends of biodegradable packing peanuts with a damp sponge. When moistened, they stick together, and you can build all kinds of unique structures.
You would think, but the children have such strong self-control at school! They have been busily building 3-D formations with marshmallows and toothpicks , and I haven't seen a single marshmallow get eaten on the school premesis - although I wouldn't hold it against anyone for having just a little taste! :)