A device that uses differences in temperature to levitate could keep small sensors floating indefinitely in the stratosphere, such as for monitoring the weather
Small sensors powered only by sunlight could float in the stratosphere indefinitely and make weather-related measurements.
There is a known effect for levitating flat objects with two sides called photophoresis. This occurs when one of the sides absorbs lots of light and the other very little, creating a difference in temperature. Just like how temperature differences in the atmosphere cause winds, this temperature gradient makes molecules move in such a way as to create a lifting force on the object.