A valance could provide just enough detail to fill the void, but not completely cover your view. I also agree with one of the previous comments, in that you could add some sort of item (i.e. plates, letters, small sculpture) which would give this space a nice focal point. Also, as you stated, the upper cabinetry is very close to the window, therefore and inside mount would be your best option. Good luck! We would love to see what you decide!