The USS Lexington is the city's most dramatic landmark: a full Essex-class aircraft carrier moored on North Beach. You can walk the flight deck, climb through tight interior spaces, and get a sense of how enormous a wartime carrier is only after you are already lost inside it.
It is a better museum than many ship attractions because the vessel itself does most of the work. Allow time, wear comfortable shoes, and expect ladders and narrow passages rather than a smooth gallery experience.