The pick your own fruit at Butler's Orchard can't be beat. This farm is very convenient to DC, and the prices are reasonable, much better than you would find at a the grocer.
There are always kids there with their parents, and the employees are very helpful. I particularly like the strawberries in May and the blueberries in July. The ride to the fields for blueberries is great, you go out in a wagon pulled by a truck. The strawberries are closer in, and you park right by them.