I can't offer plant suggestions as I am still in "beginner gardener" mode. However, I do like
@sallyg's idea of varying the height along the wall. Basically create a flowering frame around the window.
A couple thoughts that came up for me with my efforts:
I notice the nice dog in the photo. Are you concerned about toxicity? My boy will taste plants so I have to keep everything dog-safe. That limits choices - for instance siberian irises are toxic to dogs but sages aren't.
And don't forget about scents. As you can and do open the windows, a nicely fragrant plant might be quite enjoyable - unless you have allergies and then you might want to minimize that aspect.
Also consider what your intentions are with the rest of the yard. If you plan on planting trees then this space could end up in full shade in a few years. If you plan on putting in a walkway next to the flowerbed, you'll want to make sure the plants won't end up blocking the path (my bear's breech did that to me - it looks gorgeous hanging over, but I have to make a detour around it.