Is there any substance that could be harmful (that I would put in thinking it would be a good toy, I don't mean like cyanide!)?
I've always kept newspaper away from them as much as I can as I assumed the print would be poisonous, but that is based on gerbil keeping. Can a guinea pig (being much larger) handle the ink?
From what I've seen so far they seem to have awful digestive systems, the amount they seem to have to eat makes them seem like Panda bears!