You can suggest a window seat with a firm bench cushion, two back cushions, and a narrow shelf below. The visual cue is enough to create a destination for reading or looking outside.
Shape the room
Choose a cushion cover that can be removed, and leave a small gap around the window trim so air and cleaning tools can pass. Add a clip-on light for cloudy afternoons.
Let it earn its place
A made-up architectural moment can still change how a room is used, which is the part that matters.