Family Friendly: Generally flat, plenty of places to stop and check out the lake. There's a campground to stay at the park, which is nice.
This trail is great for all skill levels and is mostly flat. There are a few climbs on the north and western sides but, all in all, not very difficult. If you want to get a fast ride in with some trees whizzing past you, come to Burke Lake. If you want an easy stroll, come to Burke Lake.
For some bonus challenges, check out the powerline cut to the east of the lake (zoom out on the map). There is some fun singletrack that runs north toward Fairfax County Parkway or south to Ox Road. If you're following the Burke Lake and Lake Mercer Grand Tour
trail, it'll run perpendicular to you for some nice hill work.
Watch out for hikers and pass with care: there's plenty of dirt for smooth, rapid, accelerations.