I know you lot really wanted me to try Rubidium and Caesium reacting with water, so here is the video of Brainiac doing it. They try all of the alkali metals except Francium.
I also found this shocking archive footage of the disposal of a MASSIVE amount (20,000 pounds) of sodium in a lake after the second world war. The environmental damage this must have caused is enormous. So if you have even a ton or so of sodium you don't need any more, don't just chuck it in the Severn!
