One way to clear the clutter of all the boxes, scratching posts, and climbing trees belonging to your cats is to give them the upper walls and ceilings. You're not using those spaces anyway! It's warm up there, and they'll feel like they are up in the trees like cats should be. At least they will keep your cats from showering hair all over your upper kitchen cabinets- or not, since cats tend to accrue new territory without giving any up.
These climbing components and sleeping spots are from the German company Goldtatze, and you can see plenty of pictures of the different configurations at hauspanther. -via Metafilter