Sopot's bar scene is the liveliest in the Trójmiasto in summer, concentrated along Monte Cassino Street and the streets running parallel to it. The town has a well-established club scene — it's been a destination for Warsaw and Gdańsk weekend-trippers for decades — and in peak season the strip from the train station to the pier is continuously busy from afternoon into the small hours.
Clubs and bars in Sopot tend to be open-air or semi-open in summer; look for terraces, beer gardens, and venues where the wall between inside and outside has effectively been removed. The Ergo Arena on the Gdańsk-Sopot border hosts large concerts that keep the whole strip busy on event nights.