One of the easiest ways to visit Manassas is by train. Virginia Railway Express runs commuter service on its Manassas Line, and Amtrak trains also stop at the station in Historic Downtown.
The station puts you a short walk from downtown restaurants, shops and the museum, so a car-free day trip is easy.
Check vre.org and amtrak.com for current schedules, since service differs on weekdays and weekends.