Potomac Overlook Regional Park is one of the best kept secrets around. Here is how to get there. (When I say, "go to the end of Marcey Road," that means go as far as a normal visitor can go. Stop at the gate. The road goes on in to the park. Park at the traffic circle around the rest rooms.)
From DC or Maryland via Downtown Bridges: Take the GW Parkway upstream. Just after leaving Rosslyn, get to the left and take the Spout Run exit. The first right on Spout Run is Lorcum Lane. Turn right and take Lorcum for a mile or so to Military Road. Turn right on Military Road and go to Marcey (with the signal) and turn right. Go to the end of Marcey.
From DC or Maryland via Chain Bridge: Cross Chain Bridge into Virginia and go straight up the hill. After going under the GW Parkway, take the first right and follow the signs to get on Military Road (you make a big U-turn). Take Military Road to Marcey (with signal) and turn left. Go to the end of Marcey.
From Virginia: Find Military Road from either the north end of Glebe or from Lorcum Lane at either Spout Run or at Lee Hwy. Once on Military Road look for Marcey Road. When you get to the Military/Marcey intersection, there is only one way to turn on Marcey. Turn that way, toward the Potomac River, and go to the end of Marcey.