I finally put the crown moulding on the two windows in the guest bedroom. It definitely makes it look more finished than just the 1x6 did. Here's a reminder of the before:
And the after:
I may still add a piece of lattice moulding to the top of the crown, but probably not anytime soon. I made Roman shades using a cheap set of vinyl blinds. You can unfortunately see the glue line through the fabric, but it's not bad for about $7 a blind! (I bought a bolt of fabric eons ago for $5, and it serendipitously exactly matches the wall color - another clearance find.) The amount of sunlight coming in is hugely reduced (this room has eastern, southern, and western exposures), keeping the room substantially cooler.