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.
Here's a cool DIY idea for a cat enrichment toy. Glue together a group of toilet paper rolls inside a tissue box and randomly fill the tubes with treats. Your cat will be stimulated when they "hunt" for a treat. So simple and inexpensive.