Another subject that works extremely well by window light is macro or close-ups of flowers. You have gentle light, no wind or bugs to fight and as much time as you'd like. Like in the previous post, you can bring blooms in from your yard or a friendly neighbor who would let you visit and cut some blossoms, especially if you offer to share your results.
Having a macro lens or a close-up filter helps your camera be able to focus when closer to the flower than usual. Or stand back and use a long telephoto lens to zoom in on a portion of the flower. Remember to use a tripod or raise the ISO to ensure a sharp shot.