There's that stuff we used to call "arrow grass". Don't know the real name, but it grows long, stiff straight heads of seeds, so you can use them like darts to throw at each other. They fly pretty well and stick in pullovers (or hair). Burrs are also another popular one on the same lines! And rosehip itching powder, definitely.

Lady-pop-out-of-bed, or bindweed as it's better known. Squeeze the base of the flower just right, and the whole flower jumps off. Not hugely entertaining, but fun nonetheless.