Historic Ships in Baltimore is a collection of warships moored in the Inner Harbor, including the USS Constellation (an 1854 sloop-of-war), the USS Torsk (a Second World War submarine), the USCGC Taney (a Coast Guard cutter and the last ship afloat that was present at Pearl Harbor on December 7, 1941), and the Chesapeake lightship. A single ticket covers all vessels.
The Torsk and Taney are the most compelling — you can go below decks on both and get a visceral sense of what it was like to operate a submarine or a surface warship in the 1940s. The Constellation is more formally presented but the ship itself is extraordinary. Combined with the National Aquarium nearby, this makes for a full day on the waterfront.