Window cat door and cat aviary for inside cats.. could even keep the cat box out there.
Huge Cat Tree - From a wooden shelving unit to a cat condo... Cool! looks easy too...
outdoor cat run - I love this idea!!
Cat shelves - Or more specifically the wall scratching pad.
"Plastic door guard snaps over sharp edges of that FENCING!!! WHY didnt I think of this!!!" [Because your husband does all the wire work.]
Cat Enclosures - Outdoor Cat Runs - Many Cat Enclosure Pictures
Awesome Large DIY backyard Cat Enclosure - Cuckoo4Design
Outdoor cat enclosure!