The girls are loving these Catty Stacks from Cathousesystems. It's basically a cat tree made of cardboard boxes that you can configure in lots of different ways.
Both Yum-Yum and Pitti-Sing were in on the building.
My brother and I put the boxes together. There's a video on their website that shows you how to do it.
You can see Pitti-Sing in the green one just as soon as we put it together.
At first it was exciting for them. Then the day after they weren't so keen on it, but after a few days it became a part of their environment and now they use it all the time. I used food treats to get them to go up and down through the boxes and through the tunnel. Pitti-Sing (the orange and white) is particularly fond of tunnels so once she got the hang of it she kept going back and forth.