Dump a bag of peat moss into their run and watch what happens. Mine sprint to it and literally wallow with joy in it. I know there are some ethical questions about the way it's harvested/mined but nothing- not even meal worms- makes my girls happier. Nobody goes anywhere near the sandbox if I put peat moss in their pen. I don't even spread it around, they take care of it I don't do it often but when I do they literally writhe in it for days.