Floating Flower Tip - Most flowers float pretty well on their own until they start to pool with water and slowly sink. For a longer lasting centerpiece, use bubble wrap to make little skirts for your flowers.
Fourth of July Crafts for Kids: Flour Lawn Stars. Cut out star stencil, sprinkle grass with water (to make the flour stick) and then sprinkle with flour. This idea also works for Canada's July Birthday. Just substitute the star for a Maple Leaf.