A saltwater battery is a simple experiment that helps children understand how electricity and conductivity works. The experiment requires only a miniature light bulb, insulated copper wire, alligator clips, magnesium and iron electrodes, and a cup of saltwater, as well as a wooden base to mount the light bulb. The electrodes go in the saltwater and send voltage through a copper wire to the bulb, which lights up.
Listverse recommends a homemade volcano for an easy home experiment. The project requires a small, empty plastic soda bottle, a funnel, a couple of measuring cups, baking soda, vinegar and dish detergent. Place the bottle in the kitchen sink, insert the funnel in the top and pour the baking soda into the bottle. Then mix vinegar and dish detergent together and pour it into the bottle to make the "volcano" erupt.
This experiment takes no time at all to put together and helps children understand that some liquids are denser than others. You need dark corn syrup or honey, dishwashing liquid, vegetable oil, rubbing alcohol and water, as well as a tall glass for combining the liquids. You can also use food coloring to make it easier to see each individual liquid. Pour each liquid into the glass, one at a time, and allow them to settle and form layers. Note where each layer of liquid sits in relation to the others.
A miniature hovercraft (see Resources) makes for a good experiment as well as a toy the children will enjoy. You need a pencil, glue, paper, a spool of thread, cardboard and a balloon. Cut a 4-inch square out of the cardboard, cut a small hole in it, glue the spool on top of the hole, cover the top of the spool by gluing on a circle of paper with a hole in the middle of it; and finally blow up the balloon and stretch it over the spool. Let it go and you have a functioning mini-hovercraft.