They can get their friends to each try a different shampoo for a set length of time, ask the users for their impressions and evaluate hair quality from photos or visual examination using arbitrary scales for light reflectance, manageability, feel during application, quality impression by the user, and so on. This is not hard science but it can be made into something that requires a lot of statistical analysis and it requires good interpersonal skills. Ideally such a project would involve many people using the same shampoo to reduce the impact of the users' genetic hair quality. Trying to standardize such a project will not be easy but would certainly introduce students to the complexities of experimental design and the kind of problems cosmetics testers face. Measuring hair fiber strength under different conditions.
Hair takes on different properties depending on humidity and chemicals it has been exposed to.
You might try looking at the strength of hair that has been treated in different ways. The basic system involves tying a strand or maybe several strands of hair to a stand. At the other end tie a hook that you can hang weights on. Put a ruler by the side of the hair fiber and add the weight gradually. As you gradually add the weight, you should see the hair fiber stretch before it breaks.