Given benefits of the sun exposure / vitamin D it would be good if the solar cell could be used as a sensor for the number of minutes of daylight exposure.
This could be tracked similarly to how steps are tracked and there could be goals for daily/weekly sunlight exposure.
On a bit of a tangent it would also be good to have UV alerts as to when sunscreen / sun protection should be worn. If UV cannot be measured or proxied (e.g. high lux at these GPS coordinates means this UV exposure) by the solar cell the data could be obtained from a phone along with weather information.
