Minimalist IKEA Cabinet Hack, and lots of other ideas on hiding a litter box. Not just for the litter box but for a dog's or cat's pet bed, when company is over or just during the day, just shut the door and it won't have to be seen until bedtime.
add wood wallpaper to the kick plates of the stairs. Those rolls are expensive but you would only need one. Not practical with white treads though, it will be stuffed up from black shoe soles in no time.
Litterbox cabinet - SUCH A GOOD IDEA! Give the cats their own little litter box cubby room. It gives them their privacy and you can contain any mess on the outside of the litter box and easily clean it up.