Did you ever wonder where the idea for stain glass windows came from? Since most people could not read, the windows were meant to be a picture Bible. If you are really artsy you can paint pictures about the gospel on your windows. There is washable paint you can use. You can mix paint and soap and paint your doors and windows. If you have some money there are people that make their living painting store windows. You could hire one of these people to paint your windows.
You can put up pictures, or words on your windows. If you live in an apartment in the city this could really make an impact.
People put up signs in their windows to support sports teams. We can put signs in our windows to support Jesus.