Measuring the Rate of Photosynthesis
One of the simplest measures of photosynthesis is just measuring how much your plants grow (i.e., weigh them). Of course this technique is imperfect because of other influences on growth, the amount of time you have to wait to get a measurable change, and ideally you would want to use the dry mass of your plants to avoid water content differences, but you have opted to exclude the methods that get around those problems.