There's a lot to see and do in Baltimore. As slawecki mentioned, if your parents are into art, you should consider the Walters Art Gallery, downtown off Mount Vernon Place, and the Baltimore Museum of Art, a bit north of the city center. Both are excellent, but I prefer the Walters. The B&O Railroad Museum with its roundhouse and collection of locomotives and rolling stock is excellent and very informative. Evergreen House, a mansion from the late 1800's and furnished in the style of the Golden Age (built with B&O money) is up near Johns Hopkins University. There's also Fort McHenry, but it's a little further from downtown. Fells Point or Little Italy would be good choices for dining.