Welcome to MoCo! I live about 12 miles north of DC and always head north and/or west, including Poolesville, on my rides. There are some nice roads in western MoCo and further west towards Harper's Ferry. From there you can turn south and head back through VA (the northern end of the Shenandoah Valley) and cross the Potomac back to MD or turn north and go through PA and northern MoCo.
One specific ride could be a lap around Sugarloaf Mountain: from Poolesville take 109 north, 355 north, 80 west, 85 south (let 'er rip), 28 east back to 109.