I know this is all joking, but honestly a glider that efficiently uses thermals to stay up is extracting energy from the sun, in thermodynamic terms.
Windmilling a propeller would indeed put a tiny amount of charge back into a battery.
But the real answer is that the best use of energy is drag reduction, by a long shot. And, no, I’m not trying to gotcha anyone who’s posted so far. I just know someone will misread this thread and get the wrong idea.