Rock Creek Park has some beautiful unpaved trails, and there's always a fair number of people out on them if you're worried about being in the woods alone.

Prince William Forest Park is pretty flat as well, has a lot of trails.

Also Potomac Overlook is pretty easy too. Not super interesting hike (aside from the nature center, birds of prey, etc) but there is one trail that heads down to the Potomac.

Roosevelt Island is convenient and easy.

What exactly are you looking for? Paved, unpaved, short, long?

Thanks.