There is nothing quite so forlorn as an empty window box in winter, which is why you so often see them filled with arrangements of evergreens. However, if you prefer dried material, there are quite a few choices that should last until early spring as long as they are protected from high winds and heavy snow.
You can experiment with any plant that has an interesting outline or decorative parts. Among those with long-lasting seedpods or berries are clematis, Queen Anne’s lace, bittersweet (Celastrus scandens), roses, gas plant (Dictamnus albus), love-in-a-mist, and honesty.
Possible flowers include cockscomb, globe thistle, sea holly, globe amaranth, goldenrod, strawflower, yarrow, and many plumed grasses. My own personal favorite, though, is massed hydrangea flowers mixed with greens.
For a contrast, use silver-leaved species like dusty miller: ‘Silver King’ or ‘Silver Queen’ artemisia, or lamb’s ears.