I highly recommend everyone go to Manassas. Everyone knows about the history prior to arriving. Or at least they know what they are going to. But for me the surprise highlight was being about to walk through the battlefield on via the trails. You could literally take the same path as the confederates did from the Stone Bridge to Matthews Hill in an attempt to hold the hill. All while experiencing a great scenic walk, hike, or run. There is a number you can call from your phone to have a self guided tour or you can utilize the great knowledge of the rangers. Both are great options.
I have done this by myself and with my family and had a great time albeit different time. Kids will enjoy the nice open space to run around and explore.
Spray for ticks, wear long pants or jeans if not too hot, and check for ticks when done walking around.
Price: max $3.00 for anyone to enter.