Manassas National Battlefield Park is the marquee draw — a 5,000-acre National Park Service site preserving the fields of both First (July 1861) and Second (August 1862) Battles of Bull Run. The Henry Hill Visitor Center is the main starting point, with driving-tour loops and hiking trails past the Stone House and Deep Cut. Reenactment weekend at the Battlefield typically falls in July.
Historic Old Town Manassas centers on the restored Manassas Depot (still an active VRE station), a compact core of restaurants, shops, breweries and the Manassas Museum. The Manassas Fall Jubilee (October) closes Old Town for music and food, and the Manassas Railway Festival (June) celebrates the depot's rail heritage.
Bull Run Regional Park (about 15 minutes north) offers hiking, camping and the annual Bull Run Festival of Lights — a 2.5-mile drive-through holiday light show that runs from Thanksgiving through the New Year. Ben Lomond Historic Site preserves a Civil War-era house that served as a Confederate field hospital, and The Freedom Museum at the Manassas Regional Airport tells the story of Prince William County's military heritage.
There is more within easy reach: Manassas National Battlefield Park (Bull Run); Historic Old Town Manassas Depot and restaurants; Bull Run Festival of Lights (holiday season); Ben Lomond Historic Site; Wolf Trap National Park (~30 min northwest). Seasonal highlights: spring Battlefield hikes and Old Town patios, summer reenactment weekend and DC museum days, fall Fall Jubilee weekend, winter Bull Run Festival of Lights and off-season DC-metro rates.