Have any willow, apple or pear trees around? My bun loves chewing on branches from those. Empty toilet paper or paper towel rolls. Old phone books. Cat toy balls.
Honey loves big sheets of brown packing paper. I put a piece about a yard square in her pen & she eventually shreds it. I put it on top of her when I first give it to her & again from time to time. She seems to enjoy finding her way out from under it & I enjoy seeing her progress, by the lump she makes under the sheet.