How does caffeine affect your pulse rate or body temperature or respiration (breathing) rate? You can test the effect of a cup of coffee, caffeine pill, cola, or energy drink.
How does caffeine affect your typing speed? typing accuracy?
Does caffeine really increase the effectiveness of other pain relievers?
What effect does the presence of caffeine have on other organisms, such as daphnia, zebrafish embryo development, fruit fly activity or behavior or mutation rate, etc.
Does watering a plant with water containing caffeine have an effect on the plant? Does watering seeds with caffeinated water affect germination?
Does the method of preparing coffee (or tea) affect the total amount of caffeine in the beverage? If so, which method results in a beverage with the most/least caffeine?