This was a super easy activity that ended up being lots of fun. I filled ice cube trays half-full of water, put a foam X in each, and put them in the freezer. Once the ice was set, I added more water to cover the letters and froze them for a few more hours.
Trip was very excited to pour the ice into the bath tub.
Next time, I would probably make lager pieces of ice as these melted very quickly in the warm water, but Trip had fun anyway. He discovered that the Xs stuck to the wall and loved sticking them up, then pouring water over them and washing them away.