I have not yet been to Manassas, but on a mileage run in 1999, I used Dulles as a base for a one day visit and drove up to Harper's Ferry.

This was one of my most memorable civil war/mileage run visits. It was the last week of September, cool but not cold, and clear. The national park is basically the town itself with some wonderful displays.
It's about a 1.5 - 2.0 hour drive each way with plenty of other small towns to drive through coming and going.