We have the cardboard with the plastic ring around it that holds a ball. Daisy goes around the ring like a streak of lightning pushing the ball. Both kitties like to scratch the center.

Any box will do, but that looks like a winner to me. You could make something similar pretty easily I bet.

One thing that our kitties really like is an old wooden ladder we have in the basement. They like climbing and sitting on top. Rocky gnaws on the edge of the wood.