I would imagine you could still put climbing posts and scratching posts out there if the covering (carpet) is real short. You could put card board boxes (replace them when yucky), maybe an old tire...to hide in. Maybe some old towels/blankets for napping. Will they have shade? I think that anything you put out there will be fun for them. You could even plant some cat grass or cat nip for them!

That is really cool you are willing to provide a home for them! Keep us updated on what you decide.