Chemistry is a great area for students to test their hypotheses-creating skills. For example, a great experiment is to grow crystals from salt. The materials are readily available from around the house: borax, water, identical jars or drinking glasses, pencils and string. Boil the water and borax mixture and pour it into the jar or glass. Then tie a length of string which hangs about halfway into the container to the middle of the pencil and place the pencil over the glass. You may want to test at what temperature the crystals grow purest, or what concentration of borax grows the biggest crystal.
Physics is a second science that makes it easy to create a testable hypothesis. A great example would be to test the rate of heat exchange by finding the fastest way to cool a can of soda. By cooling room-temperature soda in a freezer, refrigerator and a bowl of ice, it is possible to calculate the rate of heat exchange and figure out which method would work best. There are lots of variations, like adding convection to the list, which you could do by placing a fan to blow on the soda.
Botany is a great introduction to the life sciences. One way to study botany is to test the effectiveness of fertilizers on tomato plants. You could test different concentrations of the same brand, or different brands (but make sure that you have a control without any fertilizer). Give the plants the same amount of light and water, and each day measure the growth of the plant. This will allow you to calculate the growth rate (in inches per day) to find out if there is a best fertilizer or optimum concentration.
Human anatomy is another branch of biology concerned with the health and well-being of people. A fine experiment would be to test different brands of soda for their corrosive properties on teeth. For this experiment, you would want to have as many tarnished pennies as brands of soda, and place them in cups with the different sodas for a set period of time (a few days is usually good). Take the pennies out of the sodas after the time has elapsed and note how much tarnish each brand took off.