Along the beach promenade, a line of wooden stalls sells smoked Baltic fish — the best and most authentic Sopot eating experience. The smoked herring (śledź) and sprats are excellent; so are the smoked mackerel and the occasional fresh-caught cod. Prices are by weight, which means you see exactly what you're getting before you pay.
The stalls tend to cluster near the pier entrance and along the seafront boardwalk. They operate whenever the weather permits from spring through early autumn, and they're busiest at lunchtime. A paper bag of smoked fish with a chunk of bread is the ideal beach lunch — informal, cheap, and very local.