I like the stretch of the W&OD from Herndon out towards ashburn. There's the Old Ox Brewery, what looks like a good barbecue place that has a mess of bicycles right by the trail a little further out, and, back towards Herndon, and Great Harvest and a nice bike shop with a coffee / smoothey / energy snack bar (Green Dragon or something like that). Sells pinarellos and asthma inhalers 
This time of year, I avoid the W&OD and the Mount Vernon trail like the plague, too crowded with too many people going too many different speeds with varying levels of awareness. I like both once the heat chases people away, and again when the cold does its thing, but this time of year, they're best for joggers and kids not idiots in aero helmets with tri bars trying to rack up koms
