engagement party spot in DC?
I don't know what time of year, but I would check out the Hotel Washington's rooftop deck. It is a beautiful location, although the food may not be incredible I have never had their plated appetizers. And it could be pricey I am not sure.
Friends recently had theirs at Tallula, which turned out really nice and wasn't very expensive as they are trying to stay on a pretty tight budget.
My Stepsister had hers at Old Ebbits and it was pretty inexpensive.
I am sure there are lots of others...