Heringsdorf is the most polished of Usedom's old Kaiserbäder, the Baltic spa resorts where Berliners came for sea air, promenades, and villa architecture. It is really three resort districts in one: Bansin in the west, Heringsdorf in the centre, and Ahlbeck by the Polish border.
The pleasure is not complicated. Walk the promenade, look up at the restored Bäderarchitektur villas, go out over the sea on Heringsdorf Pier, then keep going east to Ahlbeck Pier. The beach is wide, sandy, and long enough that the resort never feels like a single cramped strip.
Heringsdorf is worth a day if you are already on the German Baltic coast or in nearby Peenemünde. It is genteel rather than wild, but the combination of sea, architecture, and easy walking gives it a clear reason to exist as more than just another beach town.