Awww, congrats!!
Is she the type that would get embarrassed by an audience? If not the wishing well or front of the castle is always a classic.
Blue Bayou could be a cool setting too. You could step aside and speak to your server to have them bring out a "will you marry me?" dessert.
For more private settings, the water near Grizzly River Rapids in DCA may be a nice place. It can get kind of quiet over there and the water fall is so pretty.
There's also a really pretty gazebo at the DLH grounds that they actually use for weddings (I think). If no one is over there (usually the case) I think that'd also be private and romantic.