Do-it-yourself cat shelves. I seen something similar to this at the SPCA, using an assortment of shelves and large wall cubbies, and the cats loved it! You could try painting the shelves bright colors and cushioning a few for them to sleep on. kitty cats, cat furniture, cat shelves, cat climb, ikea shelv, cat trees
build your own cat furniture - IKEA Hackers
Felis Arborealis -- Gallery This cat tree was made from sugar maple, red maple and mountain laurel.
Cincinnati Cape Cod: I Built a Cat Tree
The Cats Own the Living Room (and most of the rest of the house too!)