Black Hills Park has numerous singletrack trails and park roads that can be combined into any number of loops for a day of riding.
It is really up to the rider but there is a published route that offers fun singletrack riding. Start at the visitor center near Little Seneca Lake. Head North along the East side of the lake. At about 2 miles follow the blue line trail on the right up hill. Continue climbing on Hard Rock Trail. At this time you will be paralleling I-270. Continue until reaching Old Baltimore Road and make a left. Continue about a 1/10 of a mile and look for the Field Crest Spur trail on the left. Stay right on double track then left towards Hard Rock. Take a right when you hit the blue trail and head the opposite way you climbed. Continue and at 4.8 head up to the Cabin Branch Trail. At 5.2 miles cross Lake Ridge Drive and look for the trailhead to pick up again on the other side of the road. Find the AT&T trail marked by orange. Continue until you reach Black Hill Road. Follow crossing Lake Ridge and find the singletrack you started on - turn right and head back to the visitor center.
How to get there:
From D.C. take I-270 North to Route 118 East. After a short distance, turn left on Route 355. Turn left on Old Baltimore to the park entrance. Follow Lake Ridge Drive to lot #5 near the visitor center and Little Seneca Lake.
This is a nice place to spend the day. It can be busy on weekends.