This is one of our favorite spots near DC for a short walk or hike (and also where we got engaged)! The park is free to enter for those with the National Park annual pass. Otherwise, there is a fee of $20 per vehicle. Note that there are two sides to this park - the Potomac, Maryland side (address above) and the McLean, Virginia side. Both are beautiful, but we prefer the Maryland side as it's often less crowded.
A few of our favorite (free!) places to check out in/near DC are: . Steven F. Udvar-Hazy Center . United States Botanical Garden . Hirshhorn Museum . FDR Memorial . Smithsonian National Museum of African American History and Culture . National Mall and Tidal Basin . United States National Arboretum and Bonsai Garden . Many more museums - check online for reservations for those that get more crowded
Zinnia is one of our favorite local spots for brunch/lunch/dinner. On weekends, they also offer coffee and a quick pastry to go.