Water, food coloring, plastic baggies (that can be resealed), ice cube trays, freezer.
Young children are fascinated by changes in their environment. This activity is a very simple way (especially on a hot day) to cool off while learning a little about science. Make ice cubes with water that has been colored with food dye. Best colors are red, blue, green, as those colors tend to make the brightest ice. Place the frozen ice cubes in bags and set them outside while you go about your normal playtime. Have the children periodically check the condition of the ice -- maybe place one bag of ice cubes on the ground, while one is on a table. See which one melts more quickly. Ask the children what is happening (you may be surprised with some of their answers). Let them hold the bag and "mush" the ice around so they can feel the difference between the ice and the water it has produced. If it's hot, place the bag on their hands and ask them if they are cooler now that the ice cube is touching them. Once the ice cube has completely melted, show the children that by putting the bag back in the freezer, the water will refreeze...and it will be an ice cube again!