Press some window clings on to the windows on your door. Window clings are made of either colored gel and are see through, are they are made of plastic that easily sticks to glass. They come in many different styles and designs and are widely available at retailers. They are easy to use and can be removed simply when you're ready to change them out. Seasonal clings are available as well.
Trim pretty patterned scrapbook paper or gift wrap with scissors to fit your windows. Use double-sided tape to stick it to the inside of your door so it covers the windows. The paper can be changed for the seasons, match the color of your door, or simply be a colorful way to provide privacy. Gift wrap can be used for Christmas door decorating.
Embellish the windows from the inside using small items relevant to the season. For fall, use craft adhesive to secure leaves to the glass for a festive look. Springtime can bring thoughts of flowers, so purchase artificial blooms and adhere those to the window. Christmas time calls for small ornaments or Christmas cards. Get creative and change the embellishments often.