Custom climbing shelves for your cat! I love these simple cat climbing shelves posted over at Saucy Dwellings. They used Ikea Lack shelves with a custom made shelf that wraps around the corner and leads to a window shelf over the front door.
Something a bit more elaborate than this leading up to our moon living room window? I could tie it into the tree-house theme I'm thinking of doing. It could just look like a platform tree house at the top of the "tree".