Forget the Northeastern states...Washington D.C. in the fall is absolutely gorgeous.  I may be a little biased, but I am just loving all of the trees (that seemed to change color overnight!) in Old Town Alexandria highlighting the irreplicable backdrop of historical buildings.  If your preference is to take in the leafy beauty by car, then you have to go no further than a drive on George Washington Memorial Parkway towards or from Maryland to get spectacular reds and yellows.

The true gem of the region, though, is Skyline Drive in the Shenandoah National Forest.  The north entrance is in Front Royal, Virginia - about a 1.5-2 hour drive from Washington D.C. depending on traffic and journey origin.  Plan to spend at least a few hours, especially on a peak weekend, driving the 32 miles from Front Royal to Thornton Gap, the first exit point on this 105 mile scenic drive.  Peak times for leaves can range from anywhere in mid-October to early November, depending on the summer temperatures and rainfalls.  This year, I was originally planning on going the first weekend in November, but fortunately I went to one of the "leaf watch" websites from the Department of Forestry that alerted me to the fact that this weekend the leaves were at peak.  Time to reshuffle my plans!    
I think everyone else in the Washington D.C. region also went to the same website because not only was traffic heavier on I-66 out to Front Royal, which may also be in part due to the Gold Cup races being held out in The Plains, but the line of cars at the park entrance took at least 30 minutes to get to the front where you pay your $15 single vehicle entrance fee.  When I say to allot several hours or more to doing Skyline Drive on a peak weekend, I mean it.  Traffic slows to a crawl anywhere near one of the valley overlooks.  It wasn't hard to find a parking spot once you got to the overlook because drivers were just slowing down to enjoy the views, rather than parking and getting out.
Since this wasn't my originally planned weekend, I only had the afternoon to get out there and complete the first 32 miles before coming home. With the traffic, though, I had to actually turn around after less than 10 miles! It was my observation that in the northern part of the park, at least, it seemed as if northbound traffic was significantly less crowded than the southbound lane.  So, perhaps, my advice to you (and to myself next year) is to start at Thornton Gap or even further south and go north.  I think that the Front Royal entrance is just more convenient, and therefore more popular.

Of course, no road trip great or small is complete without the best (and cutest) travel companion in the world!