I doubt you could plan to answer any outstanding (as yet unanswered) scientific question.
Perhaps looking at Newton, Faraday, Volta, etc era questions and answer them using novel means. Those guys in history didn't have the benefit of Arduinos and the enormous collection of sensors, etc.
Some examples:
\* Measure speed of sound using either speaker, microphone or ultrasonic devices.
\* Measure gravitational acceleration by dropping a weight from different heights.
\* Investigate the relationship between current, coil-turns and magnetic force.
\* Measure temperature, humidity, barometric pressure to track / predict weather.
\* Use ArduCam to take time-lapse photos of seedlings, etc?
