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.
You can easily make your own with a canvas, a paint brush, masking tape, and three colors + white acrylic paint. Just tape up your canvas every which way, paint a gradient of each color in the taped segments and remove the tape