Turn a clear glass or plastic jar into a rain gauge. Choose a jar with relatively straight sides, and have your child place it outdoors away from the eaves of your house. After it rains, use a ruler to measure the height of the water in the jar. Do this for several rainstorms, helping your child record the rainfall totals on a piece of notepaper.
If you live where it snows, it's fun to catch and examine snowflakes. Choose a dark-colored piece of cloth or construction paper so the snowflakes will show up well against it. Chill it in the refrigerator ahead of time so the snowflakes won't melt too quickly when you catch them. Have your child take this "snowflake catcher" outside and hold it up until a few flakes land on it. Help him look at them through a magnifying glass, pointing out that no two are alike.
Finally, show your child how to make his own ice. Put some water and a few drops of food coloring in a clear plastic cup or Tupperware container and place it in the freezer. Take it out when it's still half-frozen, and let your child poke his finger through the shallow layer of ice to the liquid below. Now let it freeze solid, then run warm water over the bottom of the container just long enough so the ice can slip out. Put the colored ice on a plate and let your child watch it melt.
There's a reason kids love to blow on dandelions and watch caterpillars grow into butterflies: Nothing fascinates young children more than observing nature. These simple activities give your child the chance to monitor natural processes and, in the case of making ice, to imitate it themselves. What better way to understand cause and effect?