When light objects, such as a paper boat, are placed on water, they will float. Place a drop of dish soap on the water next to the boat, and the change in surface tension will propel the boat forward. This is a basic science activity which leaves room for kids to add their own elements to it. Do they want to race their boats down a rain gutter? What happens if they design different kinds of boats, from paper boats to boats made out of Styrofoam plates? Which materials make for a faster boat? Give kids plenty of different materials and encourage them to anticipate what will happen in different situations (hypothesis) and then try it out to see if they're right.
This is another activity which actively engages kids to ask questions and then see what happens when they test it. Place several paper cups upside down on the floor, and close together. Rest a piece of cardboard on top of the cups, then stand on the cardboard to see if the cups can support a person. What happens if you move the cups further away? How many cups can you take away before it doesn't work any more? How much weight can your paper cup platform hold?
This demonstration of density is a good project for science fair projects. Begin two or more months before the fair. Fill a tall glass about 1/5 of the way full of salt. Carefully add just enough water to wet this layer of salt. Place an egg on the center of the salt. Carefully fill the glass with water, slowly pouring water down the side of the glass so the salt layer is not disturbed. Cover with clear plastic wrap and secure with a rubber band. The egg initially rests on the salt layer, but over time as the salt dissolves into the water the egg will rise. Every two weeks or so thereafter, up to the day of the fair, start another glass. By the time of the fair, you'll have several glasses at different stages of the experiment to show how it looks over time. Allow kids to experiment with variations of this. What happens if the water is poured in directly on the salt layer? Do they expect the egg to rise at the same rate every two weeks? What happens if they use a golf ball instead of an egg, or a hard-boiled egg instead of a raw one?