Experiment to see the rate of transpiration
You could use a lamp to increase the intensity of light that hits the plant - this should increase the transpiration rate. To decrease the light intensity, put the potometer in a cupboard (this should decrease the transpiration rate)
You could increase or decrease the temperature by putting the potometer in the room that's warmer or colder than where you did the control experiment. An increase in temperature should increase the transpiration rate and a decrease in temperature should lower it.
You could increase the humidity of the air around the plant by spraying a little water into a clear plastic bag before sealing it around the plant. This should decrease the rate of transpiration.
You could use a fan to increase the wind speed - this should increase the transpiration rate.