Postscript: The umbrella 'cave' is still on the living room floor, and we have been making changes to it day by day. Some crinkly packing paper is underneath it, and a toy is clipped onto the short fabric strap used to keep the umbrella closed. Some leftover colored tissue paper is hanging from a couple of the pointy rib-ends of the umbrella. Besides enjoying the sensory opportunities the cats seem to use it to play games with each other. One cat will sneak up behind the umbrella to 'tag' an unsuspecting friend. We are easily amused, eh kitties?
(The Toys section of this blog has more feline amusements! I'd love to include your favorites, if they are free or very inexpensive....send them along!)