One of the more interesting hikes in the DC area is Theodore Roosevelt Island. If you've never been there, there's a surprise in the middle of the island: a big statue of President Theodore Roosevelt. You also get some nice view of Georgetown.

When you first cross the footbridge, be sure to look at the map. There's lots of paths. To see the statue, bear right, then about 100 feet along the trail, bear left. After heavy rains, the trails can be fairly soggy, particularly on the south of the island.