Sette Bello has a large patio with outdoor seating.
Eleventh St Lounge while it doesn't have an outdoor spot for not that much moola you can rent out the whole downstairs and they have great drinks and food.
You could always cater something in a park like Bonaire or Great Falls or something or grill out?
There is a nice restaurant in Ashburn where friends had a wedding reception which was beautiful and had a huge outdoor deck, I will try to think of the name.
Sweetwater Tavern has a huge outdoor patio.
L'auberge Chez Francois does weddings in its garden area it might do parties too?
What are you looking for? Metro accessible? Doesn't matter? Price range? Ethnic food? No ethnic food?
The first place that came to mind was MeJana in Arlington (nice large outdoor patio; good food), but it may not be in your price range (probably $30 to $50 per person depending on what you order, how much you drink), may not want Lebanese food,may not want mezze style, etc. etc..