The Detroit Zoo has in my opinion by far and away the best tree kangaroo exhibit in North America, although in the wintertime I'm not sure what happens with the roos. There are photos in the gallery that show an absolutely enormous tree, and when I was in Detroit last summer the kangaroos were incredibly high up in the foliage.
The best Tree Kangaroo exhibit that I've seen is at Currumbin Wildlife Sanctuary in Queensland. It's pretty big and has a lot of climbing structures and it's in a rainforest setting.