I throw them into my trash can when I am done - they make nice filler
to make sure I am getting my money's worth from my trash collectors.
Sorry, I could just not resist. :-)

Ocassionally, in the summer, I will wash them out, fill them with water
and freeze them. When it is frozen, I give it to my dog and he lays on
it under a shade tree to keep cool. He really seems to like it.