Platforms might work great. My boy loves cat condos but my girl chews up carpeting so I had to remove them. I was thinking about getting some concrete blocks and making a maze to see if they would use it. Since this is a garden like area, they can dig anywhere they choose. They like to hide the entrance of their hole so I put a small doghouse in the middle of their yard so they won't dig next to the fence. They dig under the doghouse instead. I fill in the holes before they get too deep. It takes the two of them about a week to dig down 5 feet. If I make changes to their yard every week or two, they don't dig. I am always trying to find new things for them to explore and sometimes just moving around the tree branches does the trick, but I thought their might be some more interesting ideas out there. I also thought about making some kind of small observation tower so they could get up high and look out over the neighborhood, but I haven't figured out how to make it so it would be safe. Thanks for the ideas.