Flowers or feathers would look amazing. maybe next year's apple blossoms! Do multiple of these then frame as if the were window panes. Place a large old frame (outside frame only) around it to form a faux window.
This bright yellow window-frame provides such a vibrant pop of colour, the whole room will feel put together and complete! This would go especially nicely in a simple, all white room with a few other matching yellow accents-stunning!