Buddy Attick Lake Park is a wonderful park nestled in Old Greenbelt (easy to get to from Greenbelt Road and I-295).
There's a gorgeous lake, full of ducks and geese. The lake is also great for fishing, which is permitted.
There's a nice walking trail around the lake, and you'll usually see runners, people on bikes, couples, families, etc., making their way around.
There is a picnic area with tables and grills, a playground with swings and a slide, a basketball court, and plenty of benches to enjoy the view.