The Kittywalk "cat containment system" is set up and connected--sort of--to the pet door so there aren't any huge gaps for the cats to wiggle through. This config consists of two T-connects, a deck-and-patio section, a clubhouse and a cabana.

So far Isis likes it more than Koosh does (she still runs to the front door whenever I say, "Do you want to go outside?"), but I think that once I spend more time
actually sitting on the patio myself, instead of just shoving junk around, that they'll spend more time hanging out in it. Here is Isis grazing on cat grass like a cow.