Reverse the page for a moment, because a lot of people reading it live an hour away.
Harpers Ferry is roughly sixty-five miles from Washington and a similar distance from Baltimore, which puts it inside the range where a day trip is comfortable rather than ambitious. By car, allow an hour and a quarter to an hour and a half outside peak, and rather more inside it, and go straight to 171 Shoreline Drive rather than to the town name — the park warns that navigation apps routinely send people to the wrong one of the three places called Harpers Ferry.
The better version is the train, and it is genuinely unusual. The station sits at the West Virginia end of the railroad bridge, inside the historic lower town, so you arrive in the place rather than near it. No parking problem, no shuttle queue, and you pay the $10 per-person entrance fee rather than $20 for a vehicle.
The catch is frequency, and it needs stating plainly. MARC's Brunswick Line runs weekdays only, and it is a commuter service — most trains head toward Washington in the morning and back out in the evening, which is the wrong direction for a day-tripper going out. Amtrak's Floridian calls once in each direction daily. Both are real. Neither is turn-up-and-go. Check the current timetable with the operator and build the day around a specific train rather than the other way round.
A reasonable car-free day: arrive mid-morning, the lower town and the museums until early afternoon, the footbridge and a stretch of towpath, then Jefferson Rock and Camp Hill, and be back on the platform with time to spare. The platform is not step-free, which matters for some travellers.