I have also seen those "cat walks" and thought they were really cool. They are basically nothing more than shelves attached to the wall in strategic places so the cats can jump from one to another.
If you do an internet search for cat walks you will find them easy. They wouldn't be hard to do at all plus they take up less room than a cat tree. I have thought about running something like that around the ceiling so they can hang out up high.
Of course there is nothing better than a cat tree in front of a window! Bird and squirrel watching are a favorite past time.