Seebad Wendenschloss is a public lido on the Langer See, open since 1915. It has a sandy beach, swimming, beach volleyball and wide views across the water toward the regatta course on the opposite shore.
It is a summer stop rather than an all-weather sight, but on a warm day it gives Berlin's southeast a proper lakeside feel. Pair it with a walk in Koepenick if you want to make the trip out here more substantial.