If you want to stay close to baltimore... you can check out Patapsco Valley State Park (5 or 6 different areas) which is a greenway that extends out towards fredrick from Baltimore. (15 mins from downtown)
Liberty Resevoir has some trails around it (20-30 mins from downtown)
Further from Baltimore there is plenty of hiking in central MD...
Sugarloaf (~1 hr)
South Moutain (1.5 hrs)... annapolis rocks, black rock, washington monument state park.
If you go south (~1 hr), there is the C&O Canal & Billygoat trail which is a flat rock scramble along the potomac river, just below great falls.
If you drive 2.5 hours southwest you can endup in shenandoah national park.
North of Baltimore in addition to susquehanna valley, there is also rocks state park, and gunpowder falls.
There is plenty of stuff to do down there. I would browse the web first. My favorites (within 90 minutes) are sugarloaf, south mountain (annapolis rocks) and the billygoat trail.
Best Regards,
Justin