Kids love shadows. Try breaking out a flashlight, or removing the lampshade from a lamp in a darkened room or closet, to make shadows.
Tell a story. Consider doing some research on how to make special shapes with your fingers and tell stories about the shapes you are making. This could be a great replacement for a bedtime story!
Talk about shapes. Use some favorite toys and see what kinds of shadows the toys make. Talk about the shapes they see and how they can tell which part of the toy makes which part of the shadow. Move the object closer and farther from the light source. Try experimenting by letting them shine the light on the object in different ways. Talk about whether any of these objects allow light to shine through them.
Change colors. Add food coloring or different flavors/colors of juice to clear cups and shine the light through the cups. Notice the difference in the light you see on a white wall. Experiment with making shadows with the tinted light.
Head outdoors. Take advantage of sunshine by casting your shadow on a driveway or sidewalk. Trace the person's outline. Or, bring objects outside and check out the shadows. Consider tracing the shadow but leaving the object in place. How does the shadow change position or size throughout the day?