We hang a roll of cheap toilet paper at the top of their cage ..... it gets entertaining. Hercules nests in the igloo, so he pulls the paper into that, usually doesn't tear it, then hudini usually nests in the hammock, so he steals it from Hercules and brings it up to the hammock, so by the end of the first night we have a cage that looks like a bunch of teenagers has TP'd, but very happy, busy rats.