BLP has a great lake for fishing and boat rentals, a paved path to walk around the lake (about 5 miles) and a fantastic playground. There is a carousel and a kiddy train, frisbee golf, a regular golf course - all a short drive from DC.
We reserved campsite B14. it was right next to the bathroom (we wanted to be near the restroom for the kids) but it was about 20 ft away, so pretty distracting. The sites were oddly shaped - our grill was on the edge of the site, closer to the neighbor's picnic table. Our neighbors were all nice and respectful, but the quarters were very close.
If you are going to BLP, go for the amenities, and don't plan to spend much time at the campsite. It's an awesome park!