For easy hiking that you can cater to any length, the National Arboretum has some easy to middle hiking that is beautiful. Bonus, it's very close (e.g. inside DC).
For more challenging, the Catoctin Mountain State Park is one hour north of DC just north of Frederick. The trails are near Camp David. Pretty fun and good scenery and views. The east side trails are medium to challenging and the west side trails are challenging.
http://www.nps.gov/cato/planyourvisit/hiking.htm
For very challenging (including a rock scramble and an elevation gain of about 2500 ft), here is a hike in Shenandoah which has some beautiful vista views of Skyline Drive across the way.
http://www.hikingupward.com/SNP/OldRag/