I wanted a longer, but not too hilly, hike, someplace I had never been, but not too far away - and a loop, not an out and back. I managed all that at Manassas National Battlefield.
Battlefields aren't really my thing, which was why I hadn't made an effort to go before. I am not likely to go again, either, but it was reasonably nice. It's definitely not a place I would visit in summer; much of it is, unsurprisingly, open field, and the sun would be intense.
Luckily, it was sunny but cold. I chose to do the First and Second Manassass loops, going clockwise from the Visitor's Center, and a short loop to a historic house near the visitor's center. Both of these loops go in and out of fields and woods.
Notable points include a bridge closed on the First loop (you probably could cross it, but I took the horse ford), Stone Bridge, a stretch along Bull Run where the bluebells were starting to come out and the section around the Unfinished Railroad. If you want to see cannons, there are plenty of those too.
The hike came out to almost 11 miles, with about 650 feet of elevation gain.