As a chemist I would have to say elemental fluorine is pretty cool, although i have mot handed it. As metals go using potassium and sodium together is always fun. Never got the chance to play with caesium, as you just don't use it unless you have nothing better to do!! Bromine is fun, fuming noxious liquid that is incredibly dense, and you can set fire to magnesium and nickel catalysts pretty easily if you are not careful. Chemistry is fun!