Any window treatment with an inside mount will work....roman shade, hobbled roman shade, austrian shade, balloon shade, duette honeycomb shade, woven wood blind, roller shade which you can either trim or cover with fabric or valances. You can do venetian blinds or shutters or you can even mount drapery panels on the inside that reach the sill and tie back to one or both sides.

You always have such great information. I am wondering how a valance would work. Those corner blocks that DH is so fond of would be hidden, no?
Thank you.

Not if you mount it inside....inside mount leaves the woodwork showing.

I'm not sure where this photo originated, but I had it saved in my 'inspiration' file. Same idea as Annie has shown above.