Buy large leaves such as those of Monstera, Anthurium and Strelitzia. These give you plenty of surface to write on. Also buy a brush pen or marker in a colour that jumps off the leaf. We opted for the white 2.0 mm Pen-Touch from Sakura.
Hand-write fun quotes or the names of your guests on the leaves, or make a hashtag chain: write one letter on each leaf (and don't forget the #) and attach them to a piece of sisal string with clothes pegs. Brilliant for drawing attention to your party's hashtag!