From Webkinz Insider Wiki
If you purchase a flying pet such as a bird or bat, you can purchase Treetop Rooms on your account for 1000 KinzCash.
The Treetop Rooms were the first rooms to have moving clouds and a moving sun. Outside Yards received this feature along with weather a few months after the Treetop Rooms were introduced.
The Treetop Room floor is made of leaves and there are no walls. Instead, there is a forest and sky. Instead of doors there are clouds. When the time of day changes, so does the sky.
Contrary to what Ganz has stated in the past, there is no weather (dark clouds and rain) in Treetop Rooms.
To see other types of rooms that can be added to your Webkinz House, visit Room Types.
There is a special theme in the wShop, just for Treetop Rooms: Treetop Theme.