Sometimes the biggest statements come from the simplest arrangements. Fall is a time of great colorful harvests. Try filling a simple bowl or basket with brightly colored varieties of squash, or other harvest vegetables.
Apples are also a traditional symbol of the harvest season. Try arranging apples into a tall vase or place a row of martini glasses down the center of the table, each containing its own apple.
One of the biggest joys of autumn is the changing leaves. Bring some of this inside by collecting an assortment of different colored leaves. Scatter them across the center of the table for a simple affect, or pair them in a basket with autumn flowers. You could also repurpose a plain wreath by sticking brightly colored leaved into it and laying it flat in the center of your table, with a candle in the center.
If your gathering trends more toward Thanksgiving or Halloween, try a holiday inspired centerpiece. Carve a pumpkin and place a scented candle inside, or use the pumpkin as a temporary vase. For Thanksgiving, where table space can be limited, hang beads or strands of leaves and flowers from cake stands and pie pedestals and use your deserts as your centerpieces.