Experiment with different ways of making rock candy. Test to see which method creates largest sugar crystals the fastest. For example, saturating the string in the sugar solution, and setting it aside for a few days, makes seed crystals that enable larger crystals to form on the string. Ask taste testers if they can notice any taste or textural differences between different recipes.
Some candy manufacturers claim that their product won't melt if you hold it in your hand. Test the claim by exposing candies to different temperatures and observing what happens to the candy. For example, place several pieces of small chocolate pieces in the refrigerator, several pieces at room temperature, several pieces in the sunny window and several pieces in other warm spots. Test how much the candies melt by seeing how much force it takes to squish the candy open. Melted candies will break open easily when pressed. Solid, unmelted candies will be more difficult to press. Consider expanding the experiment by testing how different candies melt --- such as gummy and hard candies --- and comparing the similarities and differences.
Learn more about taste buds, and people's sense of smell, with taste-testing experiments on flavored candies. Ask taste testers to pinch their nose and suck on a piece of candy that they haven't seen. Ask them to identify the specific flavor using their taste buds only. Record if the taste tester's guess changes after a few moments, and how accurate the different flavors are. Compare if tasters more correctly identified one particular flavor of candy, such as watermelon, over minty or cinnamon-flavored candies.
Most peanut brittle directions specify that the recipe won't work on a humid day, since the high sugar content doesn't mix well on days over 60 percent humidity. Test making peanut brittle on a dry day, and then on a humid day, then compare what happens during the candy-making process, and how the candy turns out on a humid or a dry day. Since you'll be comparing the two results, following the recipe exactly, especially regarding mixing time, temperature and quantity of ingredients. Following the recipe exactly is important to the overall scientific process.