Walk north inside Szafarnia Street and along the marina to admire the picturesque views over boats and yachts with the Main Town and the Crane(Żuraw) in the background. If it's not too early (for you), step by at Brovarnia Gdańsk, a traditional microbrewery in a charismatic granary building, for a beer tasting.
Pay a visit to the National Maritime Museum located at the street's end. Gdansk's history is tightly connected to the Baltic Sea and the maritime culture. The museum explains this connection fantastically.
Traverse the Motlawa canal with the ferry (Tue - Sun every 15 minutes from 10 a.m. to 16.05 p.m., 1,50 PLN per person one way). It operates between June and September. Since June 2017 you can also cross the river on the footbridge behind the Baltic Philharmonic.
After this cultural infusion, walk back north along the river until you reach the Fish Square (Targ Rybny), a nice area to enjoy a seafood lunch.
For dessert, go behind the Hilton hotel to find Lody Miś, the legendary traditional ice cream parlour in Gdansk. Long queues are to be expected in July and August. Alternatively, go to Znane Smaki on parallel Tartaczna Street, where former Miś employees now make their own ice cream with use of the original recipes. Gdansk locals still debate which one is the real deal, so the choice is yours.
Go west down Podwale Staromiejskie Street. On the corner, where it touches Pańska Street, you'll see the old Market Hall. After admiring its unique construction go downstairs to look for some traditional Polish food products.
Finally, dive into the grid of the Main Town's charming streets and spend the rest of the day guided by your intuition.