A few weeks ago, I finally broke down and got my owner, Elf, a cat tree - purely in 'self-defense'.
Of course, once it was assembled an put in place, Elf wasn't quite sure what to make of it; it actually took a week of me picking her up and petting her before easing her onto one of the shelves before she was willing to start climbing up on it herself.
Weird as Elf is, I wasn't all that sure that she'd have any interest in anything like that, or whether or not she'd ever actually use it. Consequently, I didn't buy anything too elaborate or expensive; the one I got is just a basic pole with 3 horizontal shelves equally spaced between the floor and ceiling. The shelves themselves aren't that big - but Elf seems to find them acceptable, even though she does overflow them a bit.
Still, it's a major improvement over the way that she used to prefer to find a spot to nap: jumping up onto my lap, then contriving to lay down on my chest or belly, and curling up -- despite the fact that I had to support her so that she didn't begin to slide off (and use her claws to hold herself in place):