Potomac Overlook Park (but no overlook!) - Rosslyn
Potomac Overlook Park is a nice park along the George Washington Parkway above the Potomac River. The park used to have a nice view of the river, but trees on the GW Parkway have grown to all but block the view. Recently the overlook was removed, but during my visit, I walked out to the end of the trail and could just make out the river and the Georgetown Reservoir through the bare winter trees.
I spent about an hour in the park, hiking a few of the short trails, wandering through the gardens, looking at the solar panels, observing the quiet bee hives, and reading the numerous plaques and interpretive nature signs.
The highlight of the trip for me was the birds of prey, all of which appear to be injured, that are on display near the park's nature center. They have a beautiful red-tailed hawk and two owls in cages that can be viewed by the public for a few hours each day. I was the only one around, so I enjoyed the quiet with these amazing creatures.
The park has picnic areas, restrooms, and about 50 parking places. Adjacent to this park, and near the entrance, is another park that has basketball courts and other athletic facilities. The parking lots for both parks are maybe 100 yards apart on a quiet road.
Gateway Park, Rosslyn
Gateway Park stands at the Rosslyn end of the Francis Scott Key Bridge. It welcomes visitors into Virginia with three acres of green space, trails, fountains, gardens, restrooms, and an amphitheater. In September of each year this park hosts the annual Rosslyn Jazz Festival and occasional summer film festivals, and this is a prime spot to watch the fourth of July fireworks.
The Jazz Festival is held the first Saturday in September, with around four live performances lasting from about 12:30 until 7pm, and it's free.
If you want to take the Metro to get to the Roosevelt National Monument and Roosevelt Island, your best bet is to get off at Rosslyn Station and walk to this park where the trail to Roosevelt Island is marked. As far I as can tell this is the only pedestrian route to this impressive national monument from the metro. Your only other option is to drive via the George Washington Parkway.