We are planning to visit Gdansk Poland in May for 5 days. What are the must see attractions?
With five days, you might want to add two nearby towns. Oliwa has an interesting cathedral and abbot's palace. Sopot is a beach town on the Gulf of Gdansk. I'd also highly recommend a day trip to the Teutonic Knight's castle at Malbork. All of these are easily accessible by public transit. In Gdansk, I'd suggest the Central Maritime Museum. Combination tickets include the museum, the Soldek (a ship) and the medieval crane. There are multiple churches but St. Mary's Basilica is probably the most interesting. There's a lot of worthwhile architecture in the Old Town and Town Centre. Specific building of interest are Uphagens' House, Town Hall, Artus' Court, and Golden House. The Great Mill now houses a shopping center. The classic photo op is from Lead Island in front of the the Maritime Museum looking across the river toward the St. John's Gate. Leave some time for amber shopping.
The downtown Royal Way and St. Mary's Cathedral are great. I would strongly recommend Malbork Castle, about a 45 minute train ride from Gdansk, it was the Teutonic Knights capitol when they conducted their crusade against the Pagan Lithuanians back in the 1200's.