Forum Marinum is a maritime museum dedicated to Finland's seafaring history, housed in a former granary near Turku Castle. The permanent collection covers naval history, merchant shipping, and the Finnish Coast Guard, but the real draw is what's moored outside: several historic vessels including the full-rigged ship Suomen Joutsen (Swan of Finland) and the minelayer Keihässalmi are open to board.
In summer the museum complex turns into one of the most atmospheric spots in the city — you can wander the decks of a century-old sailing ship with views out toward the archipelago. Combined with the nearby castle, the harbour end of Linnankatu makes for a half-day well spent.