Crazy Cat House -I came across this article about a house in Japan that has been customised for cats. The house features cat walkways that criss cross around the walls and run overhead and cat stairs that allow the cats access to the walkways.
Remove a cabinet door panel and add a curtain with a tension rod to hide a litter box. Would be great in a laundry or mud room. Could also see if there's a way to add an actual cat door, to cut down on litter tracking.