Maryland's first-ever State Park, Patapsco Valley, is located in Catonsville, Maryland, not far from Baltimore, and about 45 minutes' drive from DC. I visited the park on a whim, in the mood for a hike, and Patapsco Valley did not disappoint.
The park's entrance is not far from I-95, and the park itself is easy to navigate thanks to the many signs, trail markers and maps available. It's a carry-in/carry-out operation, so trash cans are one thing you won't find available here - be sure to plan accordingly and bring your own bags.
My hike started on the Cascade Trail, connected with Morning Choice, then wound back to my starting point on Ridge Trail (link to trail map). The Cascade Trail appears to be among the most popular, crossing a beautiful waterfall just steps from the trailhead. I'd recommend starting early if you wish to avoid the crowds that visit specifically for the photo op it provides.
A note to extreme adventurers - this park is flanked by residential communities, which are visible from the trail at times. If you're looking for a secluded experience, this is probably not the place for you. That being said, the trails at Patapsco were lightly trafficked, extremely well marked and clean. It's especially conducive to mountain bikers, who were out in great numbers on this warm fall weekend. Overall, a really nice spot to spend day outdoors.