Manassas Battlefield has beautiful trails that venture through all kinds of different scenery. On just one of the 6 mile trails you could walk through a dense forest, empty field, dirt road, or grassy valley. The trails are relatively flat, which makes for an easy day hike. Plus, there are so many trail options, all of which have historical plaques along the way narrating events that took place during the Civil War. Since its a historical park, the Visitor Center doubles as a museum, and along the trails there are a few really interesting houses and buildings from the time period.
Most importantly dogs love it!