Whether it's a desire to be outside, interact with whatever's at the window, warming in the sun, or just watching what's going on, indoor cats are naturally drawn to spending time at the window.
Do these 3 things to give your cat a better window watching experience.
1. Get Rid Of Window Obstructions
Cats will find their way through impediments such as curtains or blinds, often making a mess of things as they do so (just ask those who have lost security deposit money or had to replace the blinds after kitty made a mess of them).
If you don't need any coverings on the window then simply remove them. If you do need coverings then find something you can open enough to give your cat an obstructed view. Don't forget to tie any cords up out of reach as well before they become play or chew toys.
2. Create A Space At The Window
Although cats will often make do in their attempt to look out the window or sleep by it, there are things that can be done to make that experience better for them.
You can simply place furniture or a cat tree or such against the window if that'll work for you, but there are also products specifically designed as a window perch or bed that you may find more appropriate instead. Many of these products attach to the window sill or attach to the window directly to create a safer and more comfortable place to lie down.
The video above shows how the window sill perch easily attaches using hook-and-loop fasteners (aka Velcro) or screws for a stronger and more permanent solution.
Be aware that there are a lot of cheaper knock-offs of this product that may not stay attached (the suction cups don't hold). If you decide to look for something cheaper then before spending money don't forget to check product reviews for any complaints of it not staying up.
3. Create Interest At The Window
You may also want to consider making your cat's window watching experience more interesting. For example, you could put a bird bath, bird feeder, squirrel feeder, etc. right in front of the window.
You can even attach a bird feeder to the outside of the window. The product above has a one-way mirror surface so although your cat can watch the birds in the feeder, the birds can't see your cat. If you're willing to forgo the one-way mirror effect, there are a lot more window attached bird feeders you can choose from.