A complete B&O trip must include the Avalon area of Patapsco Valley State Park. The park entrance takes you beneath the Thomas Viaduct, and the main park trail (paved and mostly level) follows the original B&O main line, which is still used by CSX. Here, one can still see artifacts of the very beginning of railroading in North America. This web site makes a great companion to a railfan's tour of the park: http://trainweb.org/oldmainline/ The Avalon area of the park is very convenient to Baltimore.
Not much farther away is Ellicott City, which boasts the nation's oldest remaining train station. The station contains a small museum, which is now run by Howard County, rather than the B&O museum. It is worth a look, but check the hours before going. Ellicott City also has some dining and shopping, and is an interesting old city in general.
Do the B&O museum first, though. It will help you appreciate the history when you see it in the state park and at Ellicott City.
