Make a pencil look like it bends, by putting the pencil into a glass filled with water to 2/3 capacity, and tipping it to one side. Because light appears to bend when moving into a substance with a different density, the pencil seems to bend as well.
Another simple water science project is to show the effects of different types of polarity at the molecular level. Pour a glass about 1/4 full of water, then add the same amount of vegetable oil, and watch the two separate into layers. Next, add some dishwashing liquid, and stir the mixture. The oil and water, which had separated earlier due to the different types of molecules, now mix because the dishwashing soap alters the bonds between the molecules of water and oil.
Yet another project involves gathering several glasses of different heights and widths, as well as a large measuring cup, to test your assumptions about which holds the most liquid. Line the glasses up in order, from the greatest to the least volume, on the basis of your predictions from their shapes. Fill the measuring cup up to its highest line with water. Then, pour as much water as possible into the cup that you predict holds the most. Pour the water from that into the glass you've predicted holds the second-most amount of water, to see if your assumptions are correct.