I felt bad about keeping my boys indoors, but with hungry coyotes roaming the neighborhood, I didn't have much choice.  I've tried to give them an indoor environment that would approximate the great outdoors.  With that in mind, I bought this tree house.

They love playing in it.  I didn't hang the toy... and they don't use the hammock... but they spend hours climbing on it and scratching. It's also saved my furniture. [Too bad I didn't have one at my mother's house... the boys had a great time ruining her couch.]