You could give them some seagrass mats, Theo loves digging those. For a digging box, shredded paper would be really easy to clean up. I know some people use sand or soil, but I think that would be very messy unless you had a lid on the box and you were able to clean off their feet when they were done playing in the dirt.
I think @Diane R has had digging boxes before, but I might be thinking about someone else.
I have used a tall, large plastic box with a bag of sterilised top soil in it. IF the box is tall enough it doesn't make much mess. If bunnies have trouble jumping in, you could put a step next to it.