Do NOT use cat-type clay litter, whether clumping or any other kind. The dust from the clay will make a bunny very sick.

We use Carefresh, other people use Yesterday's News - both are recycled newspapers, okay for a bunny to chew on, and not bad for them. We always put that in the litterbox, then cover it with a layer of hay - good for munching, and an incentive to stay in the litterbox. Bunnies do munch while they are "doing their business" which is yet another reason not to use clay litters.