Those look great! I'd love to have a cat tree but I don't have the funds, or the room for one right now.
Have you considered making window perches too? Or what about cat beds?
There have been some fabulous examples of crafty things made for kitties, some of them being "no sew" too. Basically just cut and tie the ends together instead of sewing.
Anything pet related will sell great.
Another idea is a raised stand for food dishes. I am currently using some plastic storage containers stacked up and turned upside down. I put Chynna's dish on top of that.
Another idea that would be good is a wall mounted scratch post. The kind that you can attach to an "outside" corner where 2 walls meet. The corners like on these islands....http://www.miseenplaceschool.com/images/kitchen.jpg
My cats don't scratch the wall, but I often see Abby stand up and stretch against a corner. If there was a scratch post high enough up, she could get in a good scratch and a stretch.