During most of the year, the Leesburg Animal Park is, well, just an "ok" place to visit for feeding small animals, checking out monkeys and porcupines, etc. (see my review of the Animal Park at http://www.trekaroo.com/activities/leesburg-animal-park-leesburg-virginia?r_id=18305.
But from the end of September through early November of each year, the Park transforms into Pumpkinville, which is a great activity the kids will love.
Pumpkinville costs only slightly more than the regular Park admission, but you also get a large playground that features hay mazes, giant hill slides, straw tunnels, moonbounces, a spooky pirate ship, all-u-can-eat fresh apples and cider, pick your own small pumpkin, etc. On weekends and holidays, you also get a 200-foot zip line, scarecrow making, mini tractor rides, face and pumpkin painting, etc. Camel rides are available at an additional cost.
If you are in northern Virginia in the fall and looking for a good way for the kids to burn off some energy, this is a good place to do it. Oh, by the way, tons of schools and groups come to Pumpkinville, so try not to time your visit for first thing in the morning.