Bonton is a kids “superstore” in Paris, which always has the most clever (and often simple) ideas in their displays. I love the metallic accordion balls they had hanging down in one window this year, and the snowflake window they did two years ago. You can buy the metallic balls for $13 a dozen, although these are 10 inches in diameter, so you have to have some pretty tall windows to do this. The snowflake one you could easily scale down to what works for your window. You could also do the accordion balls as interior decor, hanging from the ceiling over a dining table or buffet, could be really nice.
If you want to make your own hanging pom poms, or smaller scale ones for napkin rings, you can buy the metallic tissue paper here, and get the how-to here. And if you want a refresher on making snowflakes, here’s one. (I like the tip at the bottom that suggests using coffee filters.)