Take a tray of ice cubes and let the children look at them and feel how cold the tray is. Explain to them that ice is water that is really cold. Fill an empty ice tray with water and let them help you place the water filled tray in the freezer. Meanwhile take the ice out of the other tray and put the cubes in a plastic bowl. Have the kids place one cube at a time in different environments -- on a sunny window sill, outside on a snowbank in the shade and in the palm of a student's hand -- and observe what happens. Record their observations and questions. When you're through with these observations, check on the progress of the tray of water in the icebox.
Fill up a huge bucket or washtub with lukewarm water. Place it in the middle of the room and have the children make a circle around it, so everyone can see inside. Have a sack with about 10 items inside it, half that will sink and half that will float. Begin by pulling out one item that will sink and one that will float and demonstrate the difference between sinking and floating. Then pull out one item at a time and ask the children to guess whether the item will float or sink.
Fill a clear glass with carbonated water. Have the children observe the bubbles in the glass. Slowly put four or five raisins into the glass and have the children observe what the bubbles do to the raisins. The raisins will essentially "ride" the bubbles up and down. Have them make observations and ask questions.
Take four or five little sponges and place them in Petrie-like dishes with some water in the bottom. Help the students "plant" one seed in each sponge. Most any seed from a simple green garden plant will do. Place the sponges and seeds in a place with adequate light and observe them once each day, being sure to water them every day and to record the kids' observations. A variation on this same experiment is to plant the seeds in each sponge four or five days apart so they can see how the "older" shoots are taller than the younger ones.