Just to clarify,
A Dog's Day Out is actually in Merrifield, though it may have a Fairfax mailing address. It's on Gallows Road just north of Lee Highway, a little bit south of the Dunn Loring Metro station, and sort of across the street from the Merrifield Garden Center. It's in a renovated warehouse, so it's basically a gigantic area that's been subdivided into activity areas and to allow small, medium, and large dogs to be segregated by size. With economies of scale, their rates are less than $30 per day, and they have received various positive write-ups in the pet and regular press. Not exactly convenient to Arlington, unless you wanted to drive out and then Metro back into town from there. You could follow this link, then click on the Virtual Tour button to get a glimmer of what goes on...
A Dog's Day Out