YAY! more visitors to the dungeon!
Baltimore is awesome. There are a lot of little museums too like the Babe Ruth House.
Babe Ruth Birthplace and Sports Legends Museums: Babe Ruth Birthplace Museum
Poe is buried at Westminster Hall and on Halloween they have guided tours of the catacombs and story tellers in the graveyard. Awesome event and put on by the UMaB Law School
The old Baltimore Shot Tower... You'll see it while you roam around the inner harbor.
Camden Yards. Even though there isn't a game (I don't think) it really is a gorgeous park even the outside.
The Constellation is docked here in the harbor too. You can't miss it but it is a little pricey and they put up a stupid small visitor's center right in front of it so you can't see it all for the splendor that she is.
USS Constellation - Docked in the Baltimore Inner Harbor
As Greebo said, Fort McHenry (my favorite place) you can take a picnic lunch and eat on the ramparts.
And the B&O Railroad museum and roundhouse. I really enjoy this place. You can also take a ride on the oldest railroad track in American history. I'm not sure it it goes all the way to Ellicott Station or not but if you have the time for a quick mile see if you can work it in.
Baltimore & Ohio Railroad Museum