Anemones are a great choice for a black and white wedding idea. With an anemone wedding bouquet, enhance them with deep eggplant flowers like these dahlias that will "read" as black as seen in this modern bridal bouquet.
Pumpkin in a lace stocking. I've planted a bunch of mini white pumpkins. White lace stockings on them might be a nice outdoorsy Autumn touch. Inexpensive and kinda unexpected. These might be cute on the outside tables under the arbor too.
Blow up a balloon with yeast or baking soda and vinegar. Fun science project to do at home! Put vinegar and baking soda in a bottle, put the balloon on the opening, and watch the gas from the chemical reaction blow up the balloon!