I put some pieces of wood in the tub for them to chew on, and I also put some cardboard .

I think they prefer chewing on the cardboard.

also , what kind of veggie matter do you throw in for them?
do you just feed them strictly rat chow.
I tossed in some grapes and they seem to love them, I've read about others using carrots, apples. Is there anything that should be avoided?