I can't afford one of the big kitty condos, so I made one myself. We use our luggage to store out-of-season clothes, and then we piled them up at different angles so the rescue kittens I have can jump and play. The material on the soft luggage (the sort of itchy fabric) helps them cling on and they build up their muscles fast and now I have 5 strong, healthy kittens.
NOTE: make sure there are no loose loops and that the luggage is securely zipped to prevent accidents. Also, when you stack them, make sure it is sturdy enough so the mountains will never fall.