Consider a window perch for your cat. Cats love to look outside and watch what is going on around them.
There are several styles of window perches available including some that slip next the window seal and create a platform so they can look out the window. Measure your window and see what style works best. Consider a window with a "view" such as of trees, a bird feeder or people passing by. A window perch can be a simple as moving a piece of furniture that is at window height to the window and offering a comfortable bed for your cat. Before you finalize your window perch, make sure if the window opens that the screen is good and secure.
For more tips on enriching your cat's environment, please read Selecting the Right Environmental Enrichment for Your Cat