Please, throw some 500-800mile tour ideas south of DC
 

Please throw couple ideas for 500-800 bike tour south of DC. The tour can start anywhere with a good (cheap) multi-day parking (or Amtrak train station with baggage service), and can end anywhere on Amtrak train station with baggage service.
I have done Cleveland->Pittsburgh -> C&O last year, and looking for some change of scenery. Planned travel time: end of August. I live just north of DC, so preferably either a starting point or an end point should be close to DC area.

So far, I am considering
The outer banks

On the other hand, I think I would really enjoy something more inland, such as revisiting the GreenBrier river trail, but I do not see an easy way to incorporate it as a part of any bike route.

Nice scenery is preferable, some camping availability would be good.

Please, suggest

You have a couple of options.

First one is the East Coast Greenway loop. Head south on the ECG towards Raleigh and further south to Wilmington and then loop back to the colonial leg of the ECG that brings you back to Williamsburg and then over to Richmond where the Main line of the ECG runs that you came through on the initial southbound leg. If you do not want to head back north on the same ECG, you can alter it somewhat and head back north on USBR 1 to DC.

Second option is head west from DC on the W&OD as far as you can go and then head south to meet up with the Skyline Drive. Ride it all the way to where it joins the Blue Ridge Parkway. Continue south just over the North Carolina border where you can pick up NC bicycle route 4 heading east ( check out OpenCycleMap.org ). You can pick up the main trunk of the ECG heading back north...., or continue east on NC 4 to pick up the colonial leg of the ECG north, which will dump you back on the ECG or USBRS 1 bac to DC.

I understand the second option has better camping options than the first, but more hills involved. I am looking at option one myself.

I really like the Shenandoah skyline idea. And thanks for OpenCycleMap.org - google maps bike layer is not displayed adequately (or not at all) on a large scale. I will post detailed maps I worked out soon.

Something like that, maybe: Suggested tour - A bike ride in Montgomery County, MD

Check out Adventure Cycling Association, which has several mapped routes in the Southeast, including the Alleghany Mountains loop (which includes Greenbrier River trail) and Outer Banks/NC Coast loop. The inner coast of NC actually offers better riding options than the Outer Banks, which only has one highway (NC 12) for most of the route, with heavy traffic at times. The Blue Ridge Parkway and Skyline Drive together could top 500 miles, but you better like climbing. Options for food, lodging, camping and even water are limited along many stretches, and you have to plan your days carefully. The Natchez Trace in Mississippi is sort of a Southeast coastal plain version of the Blue Ridge Parkway.

America's Bicycle Travel Experts | Adventure Cycling Association

Thanks for the reply. Here is the map of what I rode: 2015 Annual ride - A bike ride in Gaithersburg, MD (I had to interrupt the tour a bit early). Some detours I took (Lurray and Shepherdstown) are not shown. Couple comments for anyone going to repeat that:
1. Stretch on 340 (miles 80 - 88) is rather unpleasant due to a lot of traffic and almost absent shoulders.
2. Route 33 (miles 158 - 162.5) is a significant descent on a winding road together with heavy trucks and all other traffic. But for the most of the descent rather wide shoulder is available. Check your brakes before attempting the descent!
3. Do not attempt the "shortcut" road on milepost 162 before reaching Lydia, VA. There might have been a road once, but now it is ovegrown with 5-6 feet vegetation with sharp thorns, the wooden bridge over Swift Run is in the bad shape, and it finally leads you in somebodies backyard, instead of being a shortcut to rt. 634.
Otherwise, a very pleasant ride, beautiful views, and last but not least: plenty of exciting people to meet on the way.

Try this route bike2dc
I've ridden it twice in the last month completely. The cue sheets are pretty good in detail.

You can take Amtrak to Charlottesville, VA but check on baggage. Several bike shops there such as Blue Wheel who can help. Blue Ridge Parkway is beautiful ride in mountains with amazing views. From Lexington, VA you can get over to Greenbriar River Trail. New River Trail is quite nice also. Bike Route 76 runs through the area.