I think the best tip to get a nice horizon is to apply a photo on a dummy wall at the exterior.
For example, to create the following image



I created a wall at the exterior of the room and changed the side texture of the wall with an imported photo. When I imported that photo as a texture, I set the same height as the one of the wall (250 cm).
A last tip : to ensure the exterior view is correctly enlighted, I put a dummy ceiling close to it (Sweet Home 3D creates a new light in the middle of each ceiling).

I created the previous image with the HotelRoom.sh3d file joined to this message.