Something my buns really love is dry sand, (clean stuff from a bag of playpit stuff, not off the beach!) They go loopy over it digging and rolling. They also seem more inclined to keep it clean than with soil or potting compost. The only thing is though that it has to during supervised play, as they occasionally try and eat it!
Still, it makes a nice change and they have a great time with it, so it's definitely recommended. G + P also like shedded paper (no ink), soil and potting compost.