Port and harbour interesting boat trips. Brilliant railroad station museum detailing pivotal role of Baltimore in the Civil War. Fantastic art gallery in the 'burbs reached by free local buses. Camden Yards baseball stadium close to downtown, Orioles fans love their pastime despite not having much of a team at present. Easy to do a day trip to DC and cheaper to stay.
We had a couple of days there as part of an eastern seaboard trip 3 summers ago. If you can get a good deal, well worth the trip.