Great Falls Park is a great place to do a day hike.  It offers access and activities as hiking, biking, horseback riding, rappelling, kayaking, and canoeing.  However go early or doing the week, the parks gets packed on the weekends.  From the NPS Site:

Great Falls Park, a site that is part of the George Washington Memorial Parkway, is an 800 acre park located along the Potomac River 14 miles upriver from Washington D.C. The park is known for two things, it's scenic beauty at the head of Potomac River fall line and the historic Patowmack Canal.¹

The falls consist of cascading rapids and several 20 foot waterfalls with a total 76 foot drop in elevation over a distance of 3500 feet. Arising from a calmer and much broader flow upstream, the Potomac constricts at Great Falls from 2500 feet just above the falls to between 60 and 100 feet along the gorge for over a mile. The Great Falls of the Potomac display the steepest and most spectacular fall line rapids of any eastern river.²

¹ National Park Service, Experience Your America, Great Falls Park, accessed on 4 August 2002, available from http://www.nps.gov/grfa/index.htm

² National Park Service, Great Falls of the Potomac, accessed on 4 August 2002, available from http://www.nps.gov/gwmp/grfa/falls/falls.htm