I have a window air conditioner behind one part of my layout. I've made removable scenery panels to allow unit operation but hide it when not needed.
Before:
(http://i51.photobucket.com/albums/f394/trainman203/image.jpg1_zpsjxjvuzwl.jpg) (http://s51.photobucket.com/user/trainman203/media/image.jpg1_zpsjxjvuzwl.jpg.html)
With trees. The tree line is low enough to allow AC unit operation.
(http://i51.photobucket.com/albums/f394/trainman203/image.jpg1_zpswbqn9vjg.jpg) (http://s51.photobucket.com/user/trainman203/media/image.jpg1_zpswbqn9vjg.jpg.html)
With sky panel (temporary, needs painting)
(http://i51.photobucket.com/albums/f394/trainman203/image.jpg1_zpsnsbconj8.jpg) (http://s51.photobucket.com/user/trainman203/media/image.jpg1_zpsnsbconj8.jpg.html)