When I was a little kid, I had a wonderful toy called the “Kenner Girder & Panel and Bridge & Turnpike Building Set”. It was like an erector set from the aliens; after building with it, you could never go back to Lincoln Logs again. (Note the bizarre mishmash of typefaces on the box, which was so typical of the early sixties.)
Turns out that some folks at Kitty City have made a similar product for building cat toys. You can construct cat buildings as huge as this one.