Ok, so it's not the most impressive thing to build, but my cat's were tearing up the furniture, so I decided to get them a cat tree. After a little bit of checking around I found out cat trees are surprisingly expensive, so I decided to build one instead.
It's kind of hard to tell from the picture due to the angle, but the whole thing is about 4' tall, with a large section underneath (with portholes) for the cats to run through/play in. I actually finished the bottom section on a previous weekend, and the cats have been loving that, and so far the top part has been finished for an hour and my cat hasn't left it since. Considering the whole thing was built with leftover wood and screws, and some cheap carpet I bought, I'd consider the whole thing a success.