Harpers Ferry up to Shepardstown and Sharpsburg (Antietam Battlefield) is nice. I did that a couple of weeks ago. This ride started at Point of Rocks, but you can zoom in to the part that goes past Harpers Ferry:
http://connect.garmin.com/activity/154540256
You could skip Shepardstown and just take Harpers Ferry Rd on into Sharpsburg. On the same route going the other direction you can see how to get to Gathland Park (South Mountain) that is also very pretty. This is all pretty hilly stuff, but I don’t think you can avoid that in this area.
You could also head into Virginia and ride around Hillsboro, Round Hill, Purcellville, Lovettsville etc.