I put bowls of water outside the night before and brought them inside in the morning. The were partially frozen. When we turned them upside down and popped the ice bowls out, they all sprang a leak and a tiny fountain of water flowed from each of them - actually, one had two holes. The kids got a huge kick out of watching through the ice as the water level inside decreased while the water leaked out. We talked about the different states of matter.
Next, the kids used water and food coloring to make different colored ice blocks. We put the color combinations in ice cube trays and froze them.
Messiest of all!
Finally, the kids painted a picture on freezer paper and then used "ice cube paint brushes" to draw designs in the painting. I froze popsicle sticks in blocks of ice the night before and popped them out in the morning for the kids to use. They loved this. After they were done, we put them out in the freezing cold to dry. The finished products are cool - they still look wet but are completely dry!