We go to the Old Maryland Farm in the Watkins Regional Park. It has some bunnies, chickens, geeses/ducks, turkeys, peacocks, cows, horses and a pig. If you go on the weekend they have a train and a merry-go-round, but that may be closed for the season. It was small, cute. There is also a farm near-by that does produce and pumpkin stuff. It is called Queen Anne's Farm, I think. I don't think it is organic, but we just went there for the pumpkins.
We have gone to the Oxon Hill farm as well, but the Old Maryland farm has a playground too. And you can feed the animals corn at the Old Maryland Farm. You can find the directions to the farms on the PG Count parks and recreation website.
I know there is a list in the library of PYO places for produce, but I gave my copy away.