Rubidium and Francium and pretty much the whole left side of the periodic table. That side swaps neutrons quite easily and therfore displace energy. Don't put Francium in Water. Rubidium in a bathtub has the same effect as a hand-grenade.
I read that ammonium nitrate when mixed with water can turn itself cold. It's used with cold packs. Perhaps by altering the concentration it can freeze?
Double-acting baking powder contains sodium bicarbonate (baking soda) and two different acid salts. When mixed with water, one of the acid salts will immediately react with the sodium bicarbonate and...