The summer retreat of emperor Wilhelm III is hidden on a small island in the Wannsee lake. The great thing about this place, listed as a UNESCO Heritage Site, is that almost everything still is as it used to be in the good old days. Especially the park remains largely unchanged. The heavily damaged castle can be visited (at an entrance fee).
Pfaueninsel can be easily combined with a day in Potsdam, though it’s gonna be a long day. The island itself will take around an hour to cover by foot, but for for example Sanssouci you need a bit more time.